[bc5b790099] Improper calculation of new dstLimit value. New test io-12.7.

2 files changed, 35 insertions, 3 deletions

diff --git a/generic/tclIO.c b/generic/tclIO.c
index 93ac937..3119db5 100644
--- a/generic/tclIO.c
+++ b/generic/tclIO.c
@@ -5632,12 +5632,11 @@ ReadChars(
 	    /* 
 	     * We read more chars than allowed.  Reset limits to
 	     * prevent that and try again.  Don't forget the extra
-	     * padding of TCL_UTF_MAX - 1 bytes demanded by the
+	     * padding of TCL_UTF_MAX bytes demanded by the
 	     * Tcl_ExternalToUtf() call!
 	     */
 
-	    dstLimit = Tcl_UtfAtIndex(dst, charsToRead + 1) 
-		    + TCL_UTF_MAX - 1 - dst;
+	    dstLimit = Tcl_UtfAtIndex(dst, charsToRead) + TCL_UTF_MAX - dst;
 	    statePtr->flags = savedFlags;
 	    statePtr->inputEncodingFlags = savedIEFlags;
 	    statePtr->inputEncodingState = savedState;
diff --git a/tests/io.test b/tests/io.test
index 7698cea..b8a5ab4 100644
--- a/tests/io.test
+++ b/tests/io.test
@@ -1478,6 +1478,39 @@ test io-12.6 {ReadChars: too many chars read} {
     }
     close $c
 } {}
+test io-12.7 {ReadChars: too many chars read [bc5b790099]} {
+    proc driver {cmd args} {
+        variable buffer
+        variable index
+        set chan [lindex $args 0]
+        switch -- $cmd {
+            initialize {
+                set index($chan) 0
+                set buffer($chan) [encoding convertto utf-8 \
+                        [string repeat \uBEEF 10]....\uBEEF]
+                return {initialize finalize watch read}
+            }
+            finalize {
+                unset index($chan) buffer($chan)
+                return
+            }
+            watch {}
+            read {
+                set n [lindex $args 1]
+                set new [expr {$index($chan) + $n}]
+                set result [string range $buffer($chan) $index($chan) $new-1]
+                set index($chan) $new
+                return $result
+            }
+        }
+    }
+    set c [chan create read [namespace which driver]]
+    chan configure $c -encoding utf-8
+    while {![eof $c]} {
+        read $c 7
+    }
+    close $c
+} {}
 
 test io-13.1 {TranslateInputEOL: cr mode} {} {
     set f [open $path(test1) w]