Merge commit 'd604b7940b14efb191a38ef22c3a38fa3adba4d0' as 'xpa'

1 files changed, 39 insertions, 0 deletions

diff --git a/xpa/BUGS b/xpa/BUGS
new file mode 100644
index 0000000..24b2a08
--- /dev/null
+++ b/xpa/BUGS
@@ -0,0 +1,39 @@
+using xpans as proxy for xpaset/xpaget (as in chandra-ed):
+
+Currently, when xpans is used as a proxy connection, an xpaset or
+xpaget command will return without waiting for the remote callback to
+complete. This is because xpans sets the mode of XPASetFd()/XPAGetFd()
+to "dofork=true", to ensure that the XPAClientLoopFork() in client.c
+is used. That routine does not wait for the callback to complete. Instead 
+it forks the callback and then "fakes" the completion. Using the forked
+loop and returning immediately prevents xpans from hanging during a long
+xpaget/xpaset callback.
+
+But starting with ds9 7.0, this causes problems: two successive xpaset calls
+which both process regions will hang the connection to ds9. For example,
+in chandra-ed's zhds9 script, a new region is generated by refinepos,
+then the old region is deleted and the new region is sent to ds9 for
+display:
+
+        # refinepos creates a new region ...
+        ${XPASET:-xpaset} -p $XPA regions deleteall
+        ${XPASET:-xpaset} $XPA regions < $OFILE
+
+This will hang the xpans connection to ds9, starting with ds9 7.0. Bill reports
+that the region parsing got slightly slower with 7.0 because he is creating
+a C++ object to pass to the lexer. Somehow that causes a race condition?
+
+To get around the problem, a delay is needed between the calls:
+
+        ${XPASET:-xpaset} -p $XPA regions deleteall
+	# this is needed to avoid a race condition, due to the fact that
+	# xpaset returns immediately when run via xpans proxy
+	sleep 1
+        ${XPASET:-xpaset} $XPA regions < $OFILE
+
+How can this be handled automatically? Ideally, one would like to
+fork() or create a thread at the "right" place in xpans to handle the
+xpaset/xpaget call so that it can wait for completion. But its unclear
+what that place is: the XPAHandler() is calling the send or receive
+callback and expects a status response, and how can fork() deal with that??
+