Fix wait_reading_process_output wait_proc hang

* src/process.c (read_process_output): Track bytes read from
a process.
(wait_reading_process_output): If called recursively through
timers and/or process filters via accept-process-output, it is
possible that the output of wait_proc has already been read by
one of those recursive calls, leaving the original call hanging
forever if no further output arrives through that fd and no
timeout has been set.  Fix that by using the process read
accounting to keep track of how many bytes have been read and
use that as a condition to break out of the infinite loop and
return to the caller as well as to calculate the proper return
value (if a wait_proc is given that is).

* src/process.h (struct Lisp_Process): Add nbytes_read to track
bytes read from a process.

(cherry picked from commit 4ba32858d61eee16f17b51aca01c15211a0912f8)

1 files changed, 2 insertions, 0 deletions

diff --git a/src/process.h b/src/process.h
index ab468b18c56..6464a8cc61a 100644
--- a/src/process.h
+++ b/src/process.h
@@ -129,6 +129,8 @@ struct Lisp_Process
     pid_t pid;
     /* Descriptor by which we read from this process.  */
     int infd;
+    /* Byte-count modulo (UINTMAX_MAX + 1) for process output read from `infd'.  */
+    uintmax_t nbytes_read;
     /* Descriptor by which we write to this process.  */
     int outfd;
     /* Descriptors that were created for this process and that need