Fix accept-process-output/process-live-p confusion

* doc/lispref/processes.texi (Accepting Output):
Document the issue.
* lisp/net/tramp-adb.el (tramp-adb-parse-device-names):
* lisp/net/tramp-rclone.el (tramp-rclone-parse-device-names):
* lisp/net/tramp-smb.el (tramp-smb-wait-for-output):
* lisp/net/tramp.el (tramp-interrupt-process):
* test/src/process-tests.el (make-process/mix-stderr):
Fix code that uses accept-process-output and process-live-p.
Add FIXME comments as necessary.
* lisp/net/tramp-sudoedit.el (tramp-sudoedit-action-sudo):
* lisp/net/tramp.el (tramp-action-out-of-band):
Add FIXME comments as necessary.

1 files changed, 20 insertions, 0 deletions

diff --git a/doc/lispref/processes.texi b/doc/lispref/processes.texi
index 72b164c5d45..afda8aede83 100644
--- a/doc/lispref/processes.texi
+++ b/doc/lispref/processes.texi
@@ -1859,6 +1859,26 @@ corresponding connection contains buffered data.  The function returns
 arrived.
 @end defun
 
+If a connection from a process contains buffered data,
+@code{accept-process-output} can return non-@code{nil} even after the
+process has exited.  Therefore, although the following loop:
+
+@example
+;; This loop contains a bug.
+(while (process-live-p process)
+  (accept-process-output process))
+@end example
+
+@noindent
+will often work, it has a race condition and can miss some output if
+@code{process-live-p} returns @code{nil} while the connection still
+contains data.  Better is to write the loop like this:
+
+@example
+(while (or (accept-process-output process)
+           (process-live-p process)))
+@end example
+
 @node Processes and Threads
 @subsection Processes and Threads
 @cindex processes, threads