diff options
Diffstat (limited to 'tcl8.6/doc/http.n')
| -rw-r--r-- | tcl8.6/doc/http.n | 818 |
1 files changed, 818 insertions, 0 deletions
diff --git a/tcl8.6/doc/http.n b/tcl8.6/doc/http.n new file mode 100644 index 0000000..7e633b3 --- /dev/null +++ b/tcl8.6/doc/http.n @@ -0,0 +1,818 @@ +'\" +'\" Copyright (c) 1995-1997 Sun Microsystems, Inc. +'\" Copyright (c) 1998-2000 by Ajuba Solutions. +'\" Copyright (c) 2004 ActiveState Corporation. +'\" +'\" See the file "license.terms" for information on usage and redistribution +'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES. +'\" +.TH "http" n 2.9 http "Tcl Bundled Packages" +.so man.macros +.BS +'\" Note: do not modify the .SH NAME line immediately below! +.SH NAME +http \- Client-side implementation of the HTTP/1.1 protocol +.SH SYNOPSIS +\fBpackage require http ?2.8?\fR +.\" See Also -useragent option documentation in body! +.sp +\fB::http::config ?\fI\-option value\fR ...? +.sp +\fB::http::geturl \fIurl\fR ?\fI\-option value\fR ...? +.sp +\fB::http::formatQuery\fR \fIkey value\fR ?\fIkey value\fR ...? +.sp +\fB::http::quoteString\fR \fIvalue\fR +.sp +\fB::http::reset\fR \fItoken\fR ?\fIwhy\fR? +.sp +\fB::http::wait \fItoken\fR +.sp +\fB::http::status \fItoken\fR +.sp +\fB::http::size \fItoken\fR +.sp +\fB::http::code \fItoken\fR +.sp +\fB::http::ncode \fItoken\fR +.sp +\fB::http::meta \fItoken\fR +.sp +\fB::http::data \fItoken\fR +.sp +\fB::http::error \fItoken\fR +.sp +\fB::http::cleanup \fItoken\fR +.sp +\fB::http::register \fIproto port command\fR +.sp +\fB::http::registerError \fIport\fR ?\fImessage\fR? +.sp +\fB::http::unregister \fIproto\fR +.BE +.SH DESCRIPTION +.PP +The \fBhttp\fR package provides the client side of the HTTP/1.1 +protocol, as defined in RFC 7230 to RFC 7235, which supersede RFC 2616. +The package implements the GET, POST, and HEAD operations +of HTTP/1.1. It allows configuration of a proxy host to get through +firewalls. The package is compatible with the \fBSafesock\fR security +policy, so it can be used by untrusted applets to do URL fetching from +a restricted set of hosts. This package can be extended to support +additional HTTP transport protocols, such as HTTPS, by providing +a custom \fBsocket\fR command, via \fB::http::register\fR. +.PP +The \fB::http::geturl\fR procedure does a HTTP transaction. +Its \fIoptions \fR determine whether a GET, POST, or HEAD transaction +is performed. +The return value of \fB::http::geturl\fR is a token for the transaction. +The value is also the name of an array in the ::http namespace +that contains state information about the transaction. The elements +of this array are described in the \fBSTATE ARRAY\fR section. +.PP +If the \fB\-command\fR option is specified, then +the HTTP operation is done in the background. +\fB::http::geturl\fR returns immediately after generating the +HTTP request and the callback is invoked +when the transaction completes. For this to work, the Tcl event loop +must be active. In Tk applications this is always true. For pure-Tcl +applications, the caller can use \fB::http::wait\fR after calling +\fB::http::geturl\fR to start the event loop. +.SH COMMANDS +.TP +\fB::http::config\fR ?\fIoptions\fR? +. +The \fB::http::config\fR command is used to set and query the name of the +proxy server and port, and the User-Agent name used in the HTTP +requests. If no options are specified, then the current configuration +is returned. If a single argument is specified, then it should be one +of the flags described below. In this case the current value of +that setting is returned. Otherwise, the options should be a set of +flags and values that define the configuration: +.RS +.TP +\fB\-accept\fR \fImimetypes\fR +. +The Accept header of the request. The default is */*, which means that +all types of documents are accepted. Otherwise you can supply a +comma-separated list of mime type patterns that you are +willing to receive. For example, +.QW "image/gif, image/jpeg, text/*" . +.TP +\fB\-pipeline\fR \fIboolean\fR +. +Specifies whether HTTP/1.1 transactions on a persistent socket will be +pipelined. See the \fBPERSISTENT SOCKETS\fR section for details. The default +is 1. +.TP +\fB\-postfresh\fR \fIboolean\fR +. +Specifies whether requests that use the \fBPOST\fR method will always use a +fresh socket, overriding the \fB-keepalive\fR option of +command \fBhttp::geturl\fR. See the \fBPERSISTENT SOCKETS\fR section for details. +The default is 0. +.TP +\fB\-proxyhost\fR \fIhostname\fR +. +The name of the proxy host, if any. If this value is the +empty string, the URL host is contacted directly. +.TP +\fB\-proxyport\fR \fInumber\fR +. +The proxy port number. +.TP +\fB\-proxyfilter\fR \fIcommand\fR +. +The command is a callback that is made during +\fB::http::geturl\fR +to determine if a proxy is required for a given host. One argument, a +host name, is added to \fIcommand\fR when it is invoked. If a proxy +is required, the callback should return a two-element list containing +the proxy server and proxy port. Otherwise the filter should return +an empty list. The default filter returns the values of the +\fB\-proxyhost\fR and \fB\-proxyport\fR settings if they are +non-empty. +.TP +\fB\-repost\fR \fIboolean\fR +. +Specifies what to do if a POST request over a persistent connection fails +because the server has half-closed the connection. If boolean \fBtrue\fR, the +request +will be automatically retried; if boolean \fBfalse\fR it will not, and the +application +that uses \fBhttp::geturl\fR is expected to seek user confirmation before +retrying the POST. The value \fBtrue\fR should be used only under certain +conditions. See the \fBPERSISTENT SOCKETS\fR section for details. The +default is 0. +.TP +\fB\-urlencoding\fR \fIencoding\fR +. +The \fIencoding\fR used for creating the x-url-encoded URLs with +\fB::http::formatQuery\fR and \fB::http::quoteString\fR. +The default is \fButf-8\fR, as specified by RFC +2718. Prior to http 2.5 this was unspecified, and that behavior can be +returned by specifying the empty string (\fB{}\fR), although +\fIiso8859-1\fR is recommended to restore similar behavior but without the +\fB::http::formatQuery\fR or \fB::http::quoteString\fR +throwing an error processing non-latin-1 characters. +.TP +\fB\-useragent\fR \fIstring\fR +. +The value of the User-Agent header in the HTTP request. In an unsafe +interpreter, the default value depends upon the operating system, and +the version numbers of \fBhttp\fR and \fBTcl\fR, and is (for example) +.QW "\fBMozilla/5.0 (Windows; U; Windows NT 10.0) http/2.8.12 Tcl/8.6.8\fR" . +A safe interpreter cannot determine its operating system, and so the default +in a safe interpreter is to use a Windows 10 value with the current version +numbers of \fBhttp\fR and \fBTcl\fR. +.TP +\fB\-zip\fR \fIboolean\fR +. +If the value is boolean \fBtrue\fR, then by default requests will send a header +.QW "\fBAccept-Encoding: gzip,deflate,compress\fR" . +If the value is boolean \fBfalse\fR, then by default this header will not be sent. +In either case the default can be overridden for an individual request by +supplying a custom \fBAccept-Encoding\fR header in the \fB-headers\fR option +of \fBhttp::geturl\fR. The default is 1. +.RE +.TP +\fB::http::geturl\fR \fIurl\fR ?\fIoptions\fR? +. +The \fB::http::geturl\fR command is the main procedure in the package. +The \fB\-query\fR option causes a POST operation and +the \fB\-validate\fR option causes a HEAD operation; +otherwise, a GET operation is performed. The \fB::http::geturl\fR command +returns a \fItoken\fR value that can be used to get +information about the transaction. See the \fBSTATE ARRAY\fR and +\fBERRORS\fR section for +details. The \fB::http::geturl\fR command blocks until the operation +completes, unless the \fB\-command\fR option specifies a callback +that is invoked when the HTTP transaction completes. +\fB::http::geturl\fR takes several options: +.RS +.TP +\fB\-binary\fR \fIboolean\fR +. +Specifies whether to force interpreting the URL data as binary. Normally +this is auto-detected (anything not beginning with a \fBtext\fR content +type or whose content encoding is \fBgzip\fR or \fBcompress\fR is +considered binary data). +.TP +\fB\-blocksize\fR \fIsize\fR +. +The block size used when reading the URL. +At most \fIsize\fR bytes are read at once. After each block, a call to the +\fB\-progress\fR callback is made (if that option is specified). +.TP +\fB\-channel\fR \fIname\fR +. +Copy the URL contents to channel \fIname\fR instead of saving it in +\fBstate(body)\fR. +.TP +\fB\-command\fR \fIcallback\fR +. +Invoke \fIcallback\fR after the HTTP transaction completes. +This option causes \fB::http::geturl\fR to return immediately. +The \fIcallback\fR gets an additional argument that is the \fItoken\fR returned +from \fB::http::geturl\fR. This token is the name of an array that is +described in the \fBSTATE ARRAY\fR section. Here is a template for the +callback: +.RS +.PP +.CS +proc httpCallback {token} { + upvar #0 $token state + # Access state as a Tcl array +} +.CE +.RE +.TP +\fB\-handler\fR \fIcallback\fR +. +Invoke \fIcallback\fR whenever HTTP data is available; if present, nothing +else will be done with the HTTP data. This procedure gets two additional +arguments: the socket for the HTTP data and the \fItoken\fR returned from +\fB::http::geturl\fR. The token is the name of a global array that is +described in the \fBSTATE ARRAY\fR section. The procedure is expected +to return the number of bytes read from the socket. Here is a +template for the callback: +.RS +.PP +.CS +proc httpHandlerCallback {socket token} { + upvar #0 $token state + # Access socket, and state as a Tcl array + # For example... + ... + set data [read $socket 1000] + set nbytes [string length $data] + ... + return $nbytes +} +.CE +.RE +.TP +\fB\-headers\fR \fIkeyvaluelist\fR +. +This option is used to add headers not already specified +by \fB::http::config\fR to the HTTP request. The +\fIkeyvaluelist\fR argument must be a list with an even number of +elements that alternate between keys and values. The keys become +header field names. Newlines are stripped from the values so the +header cannot be corrupted. For example, if \fIkeyvaluelist\fR is +\fBPragma no-cache\fR then the following header is included in the +HTTP request: +.RS +.PP +.CS +Pragma: no-cache +.CE +.RE +.TP +\fB\-keepalive\fR \fIboolean\fR +. +If boolean \fBtrue\fR, attempt to keep the connection open for servicing +multiple requests. Default is 0. +.TP +\fB\-method\fR \fItype\fR +. +Force the HTTP request method to \fItype\fR. \fB::http::geturl\fR will +auto-select GET, POST or HEAD based on other options, but this option +enables choices like PUT and DELETE for webdav support. +.TP +\fB\-myaddr\fR \fIaddress\fR +. +Pass an specific local address to the underlying \fBsocket\fR call in case +multiple interfaces are available. +.TP +\fB\-progress\fR \fIcallback\fR +. +The \fIcallback\fR is made after each transfer of data from the URL. +The callback gets three additional arguments: the \fItoken\fR from +\fB::http::geturl\fR, the expected total size of the contents from the +\fBContent-Length\fR meta-data, and the current number of bytes +transferred so far. The expected total size may be unknown, in which +case zero is passed to the callback. Here is a template for the +progress callback: +.RS +.PP +.CS +proc httpProgress {token total current} { + upvar #0 $token state +} +.CE +.RE +.TP +\fB\-protocol\fR \fIversion\fR +. +Select the HTTP protocol version to use. This should be 1.0 or 1.1 (the +default). Should only be necessary for servers that do not understand or +otherwise complain about HTTP/1.1. +.TP +\fB\-query\fR \fIquery\fR +. +This flag causes \fB::http::geturl\fR to do a POST request that passes the +\fIquery\fR to the server. The \fIquery\fR must be an x-url-encoding +formatted query. The \fB::http::formatQuery\fR procedure can be used to +do the formatting. +.TP +\fB\-queryblocksize\fR \fIsize\fR +. +The block size used when posting query data to the URL. +At most +\fIsize\fR +bytes are written at once. After each block, a call to the +\fB\-queryprogress\fR +callback is made (if that option is specified). +.TP +\fB\-querychannel\fR \fIchannelID\fR +. +This flag causes \fB::http::geturl\fR to do a POST request that passes the +data contained in \fIchannelID\fR to the server. The data contained in +\fIchannelID\fR must be an x-url-encoding +formatted query unless the \fB\-type\fR option below is used. +If a Content-Length header is not specified via the \fB\-headers\fR options, +\fB::http::geturl\fR attempts to determine the size of the post data +in order to create that header. If it is +unable to determine the size, it returns an error. +.TP +\fB\-queryprogress\fR \fIcallback\fR +. +The \fIcallback\fR is made after each transfer of data to the URL +(i.e. POST) and acts exactly like the \fB\-progress\fR option (the +callback format is the same). +.TP +\fB\-strict\fR \fIboolean\fR +. +Whether to enforce RFC 3986 URL validation on the request. Default is 1. +.TP +\fB\-timeout\fR \fImilliseconds\fR +. +If \fImilliseconds\fR is non-zero, then \fB::http::geturl\fR sets up a timeout +to occur after the specified number of milliseconds. +A timeout results in a call to \fB::http::reset\fR and to +the \fB\-command\fR callback, if specified. +The return value of \fB::http::status\fR is \fBtimeout\fR +after a timeout has occurred. +.TP +\fB\-type\fR \fImime-type\fR +. +Use \fImime-type\fR as the \fBContent-Type\fR value, instead of the +default value (\fBapplication/x-www-form-urlencoded\fR) during a +POST operation. +.TP +\fB\-validate\fR \fIboolean\fR +. +If \fIboolean\fR is non-zero, then \fB::http::geturl\fR does an HTTP HEAD +request. This request returns meta information about the URL, but the +contents are not returned. The meta information is available in the +\fBstate(meta) \fR variable after the transaction. See the +\fBSTATE ARRAY\fR section for details. +.RE +.TP +\fB::http::formatQuery\fR \fIkey value\fR ?\fIkey value\fR ...? +. +This procedure does x-url-encoding of query data. It takes an even +number of arguments that are the keys and values of the query. It +encodes the keys and values, and generates one string that has the +proper & and = separators. The result is suitable for the +\fB\-query\fR value passed to \fB::http::geturl\fR. +.TP +\fB::http::quoteString\fR \fIvalue\fR +. +This procedure does x-url-encoding of string. It takes a single argument and +encodes it. +.TP +\fB::http::reset\fR \fItoken\fR ?\fIwhy\fR? +. +This command resets the HTTP transaction identified by \fItoken\fR, if any. +This sets the \fBstate(status)\fR value to \fIwhy\fR, which defaults to +\fBreset\fR, and then calls the registered \fB\-command\fR callback. +.TP +\fB::http::wait\fR \fItoken\fR +. +This is a convenience procedure that blocks and waits for the +transaction to complete. This only works in trusted code because it +uses \fBvwait\fR. Also, it is not useful for the case where +\fB::http::geturl\fR is called \fIwithout\fR the \fB\-command\fR option +because in this case the \fB::http::geturl\fR call does not return +until the HTTP transaction is complete, and thus there is nothing to +wait for. +.TP +\fB::http::data\fR \fItoken\fR +. +This is a convenience procedure that returns the \fBbody\fR element +(i.e., the URL data) of the state array. +.TP +\fB::http::error\fR \fItoken\fR +. +This is a convenience procedure that returns the \fBerror\fR element +of the state array. +.TP +\fB::http::status\fR \fItoken\fR +. +This is a convenience procedure that returns the \fBstatus\fR element of +the state array. +.TP +\fB::http::code\fR \fItoken\fR +. +This is a convenience procedure that returns the \fBhttp\fR element of the +state array. +.TP +\fB::http::ncode\fR \fItoken\fR +. +This is a convenience procedure that returns just the numeric return +code (200, 404, etc.) from the \fBhttp\fR element of the state array. +.TP +\fB::http::size\fR \fItoken\fR +. +This is a convenience procedure that returns the \fBcurrentsize\fR +element of the state array, which represents the number of bytes +received from the URL in the \fB::http::geturl\fR call. +.TP +\fB::http::meta\fR \fItoken\fR +. +This is a convenience procedure that returns the \fBmeta\fR +element of the state array which contains the HTTP response +headers. See below for an explanation of this element. +.TP +\fB::http::cleanup\fR \fItoken\fR +. +This procedure cleans up the state associated with the connection +identified by \fItoken\fR. After this call, the procedures +like \fB::http::data\fR cannot be used to get information +about the operation. It is \fIstrongly\fR recommended that you call +this function after you are done with a given HTTP request. Not doing +so will result in memory not being freed, and if your app calls +\fB::http::geturl\fR enough times, the memory leak could cause a +performance hit...or worse. +.TP +\fB::http::register\fR \fIproto port command\fR +. +This procedure allows one to provide custom HTTP transport types +such as HTTPS, by registering a prefix, the default port, and the +command to execute to create the Tcl \fBchannel\fR. E.g.: +.RS +.PP +.CS +package require http +package require tls + +::http::register https 443 ::tls::socket + +set token [::http::geturl https://my.secure.site/] +.CE +.RE +.TP +\fB::http::registerError\fR \fIport\fR ?\fImessage\fR? +. +This procedure allows a registered protocol handler to deliver an error +message for use by \fBhttp\fR. Calling this command does not raise an +error. The command is useful when a registered protocol detects an problem +(for example, an invalid TLS certificate) that will cause an error to +propagate to \fBhttp\fR. The command allows \fBhttp\fR to provide a +precise error message rather than a general one. The command returns the +value provided by the last call with argument \fImessage\fR, or the empty +string if no such call has been made. +.TP +\fB::http::unregister\fR \fIproto\fR +. +This procedure unregisters a protocol handler that was previously +registered via \fB::http::register\fR, returning a two-item list of +the default port and handler command that was previously installed +(via \fB::http::register\fR) if there was such a handler, and an error if +there was no such handler. +.SH ERRORS +The \fB::http::geturl\fR procedure will raise errors in the following cases: +invalid command line options, +an invalid URL, +a URL on a non-existent host, +or a URL at a bad port on an existing host. +These errors mean that it +cannot even start the network transaction. +It will also raise an error if it gets an I/O error while +writing out the HTTP request header. +For synchronous \fB::http::geturl\fR calls (where \fB\-command\fR is +not specified), it will raise an error if it gets an I/O error while +reading the HTTP reply headers or data. Because \fB::http::geturl\fR +does not return a token in these cases, it does all the required +cleanup and there is no issue of your app having to call +\fB::http::cleanup\fR. +.PP +For asynchronous \fB::http::geturl\fR calls, all of the above error +situations apply, except that if there is any error while reading the +HTTP reply headers or data, no exception is thrown. This is because +after writing the HTTP headers, \fB::http::geturl\fR returns, and the +rest of the HTTP transaction occurs in the background. The command +callback can check if any error occurred during the read by calling +\fB::http::status\fR to check the status and if its \fIerror\fR, +calling \fB::http::error\fR to get the error message. +.PP +Alternatively, if the main program flow reaches a point where it needs +to know the result of the asynchronous HTTP request, it can call +\fB::http::wait\fR and then check status and error, just as the +callback does. +.PP +In any case, you must still call +\fB::http::cleanup\fR to delete the state array when you are done. +.PP +There are other possible results of the HTTP transaction +determined by examining the status from \fB::http::status\fR. +These are described below. +.TP +\fBok\fR +. +If the HTTP transaction completes entirely, then status will be \fBok\fR. +However, you should still check the \fB::http::code\fR value to get +the HTTP status. The \fB::http::ncode\fR procedure provides just +the numeric error (e.g., 200, 404 or 500) while the \fB::http::code\fR +procedure returns a value like +.QW "HTTP 404 File not found" . +.TP +\fBeof\fR +. +If the server closes the socket without replying, then no error +is raised, but the status of the transaction will be \fBeof\fR. +.TP +\fBerror\fR +. +The error message will also be stored in the \fBerror\fR status +array element, accessible via \fB::http::error\fR. +.PP +Another error possibility is that \fB::http::geturl\fR is unable to +write all the post query data to the server before the server +responds and closes the socket. +The error message is saved in the \fBposterror\fR status array +element and then \fB::http::geturl\fR attempts to complete the +transaction. +If it can read the server's response +it will end up with an \fBok\fR status, otherwise it will have +an \fBeof\fR status. +.SH "STATE ARRAY" +The \fB::http::geturl\fR procedure returns a \fItoken\fR that can be used to +get to the state of the HTTP transaction in the form of a Tcl array. +Use this construct to create an easy-to-use array variable: +.PP +.CS +upvar #0 $token state +.CE +.PP +Once the data associated with the URL is no longer needed, the state +array should be unset to free up storage. +The \fB::http::cleanup\fR procedure is provided for that purpose. +The following elements of +the array are supported: +.RS +.TP +\fBbinary\fR +. +This is boolean \fBtrue\fR if (after decoding any compression specified +by the +.QW "Content-Encoding" +response header) the HTTP response is binary. It is boolean \fBfalse\fR +if the HTTP response is text. +.TP +\fBbody\fR +. +The contents of the URL. This will be empty if the \fB\-channel\fR +option has been specified. This value is returned by the \fB::http::data\fR command. +.TP +\fBcharset\fR +. +The value of the charset attribute from the \fBContent-Type\fR meta-data +value. If none was specified, this defaults to the RFC standard +\fBiso8859-1\fR, or the value of \fB$::http::defaultCharset\fR. Incoming +text data will be automatically converted from this charset to utf-8. +.TP +\fBcoding\fR +. +A copy of the \fBContent-Encoding\fR meta-data value. +.TP +\fBcurrentsize\fR +. +The current number of bytes fetched from the URL. +This value is returned by the \fB::http::size\fR command. +.TP +\fBerror\fR +. +If defined, this is the error string seen when the HTTP transaction +was aborted. +.TP +\fBhttp\fR +. +The HTTP status reply from the server. This value +is returned by the \fB::http::code\fR command. The format of this value is: +.RS +.PP +.CS +\fIHTTP/1.1 code string\fR +.CE +.PP +The \fIcode\fR is a three-digit number defined in the HTTP standard. +A code of 200 is OK. Codes beginning with 4 or 5 indicate errors. +Codes beginning with 3 are redirection errors. In this case the +\fBLocation\fR meta-data specifies a new URL that contains the +requested information. +.RE +.TP +\fBmeta\fR +. +The HTTP protocol returns meta-data that describes the URL contents. +The \fBmeta\fR element of the state array is a list of the keys and +values of the meta-data. This is in a format useful for initializing +an array that just contains the meta-data: +.RS +.PP +.CS +array set meta $state(meta) +.CE +.PP +Some of the meta-data keys are listed below, but the HTTP standard defines +more, and servers are free to add their own. +.TP +\fBContent-Type\fR +. +The type of the URL contents. Examples include \fBtext/html\fR, +\fBimage/gif,\fR \fBapplication/postscript\fR and +\fBapplication/x-tcl\fR. +.TP +\fBContent-Length\fR +. +The advertised size of the contents. The actual size obtained by +\fB::http::geturl\fR is available as \fBstate(currentsize)\fR. +.TP +\fBLocation\fR +. +An alternate URL that contains the requested data. +.RE +.TP +\fBposterror\fR +. +The error, if any, that occurred while writing +the post query data to the server. +.TP +\fBstatus\fR +. +Either \fBok\fR, for successful completion, \fBreset\fR for +user-reset, \fBtimeout\fR if a timeout occurred before the transaction +could complete, or \fBerror\fR for an error condition. During the +transaction this value is the empty string. +.TP +\fBtotalsize\fR +. +A copy of the \fBContent-Length\fR meta-data value. +.TP +\fBtype\fR +. +A copy of the \fBContent-Type\fR meta-data value. +.TP +\fBurl\fR +. +The requested URL. +.RE +.SH "PERSISTENT CONNECTIONS" +.PP +.SS "BASICS" +.PP +See RFC 7230 Sec 6, which supersedes RFC 2616 Sec 8.1. +.PP +A persistent connection allows multiple HTTP/1.1 transactions to be +carried over the same TCP connection. Pipelining allows a +client to make multiple requests over a persistent connection without +waiting for each response. The server sends responses in the same order +that the requests were received. +.PP +If a POST request fails to complete, typically user confirmation is +needed before sending the request again. The user may wish to verify +whether the server was modified by the failed POST request, before +sending the same request again. +.PP +A HTTP request will use a persistent socket if the call to +\fBhttp::geturl\fR has the option \fB-keepalive true\fR. It will use +pipelining where permitted if the \fBhttp::config\fR option +\fB-pipeline\fR is boolean \fBtrue\fR (its default value). +.PP +The http package maintains no more than one persistent connection to each +server (i.e. each value of +.QW "domain:port" ). +If \fBhttp::geturl\fR is called to make a request over a persistent +connection while the connection is busy with another request, the new +request will be held in a queue until the connection is free. +.PP +The http package does not support HTTP/1.0 persistent connections +controlled by the \fBKeep-Alive\fR header. +.SS "SPECIAL CASES" +.PP +This subsection discusses issues related to closure of the +persistent connection by the server, automatic retry of failed requests, +the special treatment necessary for POST requests, and the options for +dealing with these cases. +.PP +In accordance with RFC 7230, \fBhttp::geturl\fR does not pipeline +requests that use the POST method. If a POST uses a persistent +connection and is not the first request on that connection, +\fBhttp::geturl\fR waits until it has received the response for the previous +request; or (if \fBhttp::config\fR option \fB-postfresh\fR is boolean \fBtrue\fR) it +uses a new connection for each POST. +.PP +If the server is processing a number of pipelined requests, and sends a +response header +.QW "\fBConnection: close\fR" +with one of the responses (other than the last), then subsequent responses +are unfulfilled. \fBhttp::geturl\fR will send the unfulfilled requests again +over a new connection. +.PP +A difficulty arises when a HTTP client sends a request over a persistent +connection that has been idle for a while. The HTTP server may +half-close an apparently idle connection while the client is sending a +request, but before the request arrives at the server: in this case (an +.QW "asynchronous close event" ) +the request will fail. The difficulty arises because the client cannot +be certain whether the POST modified the state of the server. For HEAD or +GET requests, \fBhttp::geturl\fR opens another connection and retransmits +the failed request. However, if the request was a POST, RFC 7230 forbids +automatic retry by default, suggesting either user confirmation, or +confirmation by user-agent software that has semantic understanding of +the application. The \fBhttp::config\fR option \fB-repost\fR allows for +either possibility. +.PP +Asynchronous close events can occur only in a short interval of time. The +\fBhttp\fR package monitors each persistent connection for closure by the +server. Upon detection, the connection is also closed at the client end, +and subsequent requests will use a fresh connection. +.PP +If the \fBhttp::geturl\fR command is called with option \fB-keepalive true\fR, +then it will both try to use an existing persistent connection +(if one is available), and it will send the server a +.QW "\fBConnection: keep-alive\fR" +request header asking to keep the connection open for future requests. +.PP +The \fBhttp::config\fR options \fB-pipeline\fR, \fB-postfresh\fR, and +\fB-repost\fR relate to persistent connections. +.PP +Option \fB-pipeline\fR, if boolean \fBtrue\fR, will pipeline GET and HEAD requests +made +over a persistent connection. POST requests will not be pipelined - if the +POST is not the first transaction on the connection, its request will not +be sent until the previous response has finished. GET and HEAD requests +made after a POST will not be sent until the POST response has been +delivered, and will not be sent if the POST fails. +.PP +Option \fB-postfresh\fR, if boolean \fBtrue\fR, will override the \fBhttp::geturl\fR option +\fB-keepalive\fR, and always open a fresh connection for a POST request. +.PP +Option \fB-repost\fR, if \fBtrue\fR, permits automatic retry of a POST request +that fails because it uses a persistent connection that the server has +half-closed (an +.QW "asynchronous close event" ). +Subsequent GET and HEAD requests in a failed pipeline will also be retried. +\fIThe -repost option should be used only if the application understands +that the retry is appropriate\fR - specifically, the application must know +that if the failed POST successfully modified the state of the server, a repeat POST +would have no adverse effect. +.SH EXAMPLE +.PP +This example creates a procedure to copy a URL to a file while printing a +progress meter, and prints the meta-data associated with the URL. +.PP +.CS +proc httpcopy { url file {chunk 4096} } { + set out [open $file w] + set token [\fB::http::geturl\fR $url -channel $out \e + -progress httpCopyProgress -blocksize $chunk] + close $out + + # This ends the line started by httpCopyProgress + puts stderr "" + + upvar #0 $token state + set max 0 + foreach {name value} $state(meta) { + if {[string length $name] > $max} { + set max [string length $name] + } + if {[regexp -nocase ^location$ $name]} { + # Handle URL redirects + puts stderr "Location:$value" + return [httpcopy [string trim $value] $file $chunk] + } + } + incr max + foreach {name value} $state(meta) { + puts [format "%-*s %s" $max $name: $value] + } + + return $token +} +proc httpCopyProgress {args} { + puts -nonewline stderr . + flush stderr +} +.CE +.SH "SEE ALSO" +safe(n), socket(n), safesock(n) +.SH KEYWORDS +internet, security policy, socket, www +'\" Local Variables: +'\" mode: nroff +'\" End: |