[PATCH] Do not skip jobserver flags with negative file descriptors

[Senthil Kumar]

Hello,

    As part of jobserver detection, the jobserver_info constructor looks
    for the last occurence of "--jobserver-auth=", and parses the argument
    for that option to look for either a named pipe (has a "fifo:" prefix)
    or read and write pipe fds separated by a comma.

    With GNU Make 4.4 and above, named pipes are used by default because
    anonymous pipe fds cannot be used reliably (see
    https://savannah.gnu.org/bugs/?57242), but it still supports a
    --jobserver-style=pipe option for compatibility reasons. With that
    flag enabled, or with older GNU Make versions, the jobserver detection
    code checks if the provided fds are greater than 0 and valid. If
    invalid, it drops them from MAKEFLAGS used for subsequent child
    processes.

    However, GNU Make explicitly provides negative jobserver pipe fds if
    the command is not marked as recursive (i.e is missing a '+' prefix in
    the Makefile) - see https://savannah.gnu.org/bugs/?57242#comment13.

    The MAKEFLAGS look like this in that case.

    -j4 --jobserver-auth=3,4 --jobserver-auth=-2,-2

    Stripping off the final --jobserver-auth with negative fds undoes this
    helpful hint from GNU Make, and exposes child processes (like
    lto-wrapper) to all the issues with anonymous pipe fds that forced GNU
    Make to switch to named pipes by default.

    This patch prevents that stripping if jobserver communication is *not*
    via a named pipe *and* file descriptors are negative. As a result,
    child processes receive MAKEFLAGS with negative file descriptors too,
    and correctly decide that a jobserver is not available.

    Ok for master?

Regards
Senthil

gcc/ChangeLog:

    * opts-common.cc (jobserver_info::jobserver_info): Do not strip jobserver
    auth flags with negative file descriptors.

diff --git gcc/opts-common.cc gcc/opts-common.cc
index 379402e68a6..aaed63d1344 100644
--- gcc/opts-common.cc
+++ gcc/opts-common.cc
@@ -2110,7 +2110,8 @@ jobserver_info::jobserver_info ()
       if (n != string::npos)
     {
       string ending = makeflags.substr (n + js_needle.size ());
-      if (ending.find (fifo_prefix) == 0)
+      bool is_named_pipe = ending.find (fifo_prefix) == 0;
+      if (is_named_pipe)
         {
           ending = ending.substr (fifo_prefix.size ());
           pipe_path = ending.substr (0, ending.find (' '));
@@ -2125,11 +2126,15 @@ jobserver_info::jobserver_info ()
         is_active = true;
       else
         {
-          string dup = makeflags.substr (0, n);
-          size_t pos = makeflags.find (' ', n);
-          if (pos != string::npos)
-        dup += makeflags.substr (pos);
-          skipped_makeflags = "MAKEFLAGS=" + dup;
+          bool negative_fds = !is_named_pipe && rfd < 0 && wfd < 0;
+          if (!negative_fds)
+            {
+              string dup = makeflags.substr (0, n);
+              size_t pos = makeflags.find (' ', n);
+              if (pos != string::npos)
+                dup += makeflags.substr (pos);
+              skipped_makeflags = "MAKEFLAGS=" + dup;
+            }
           error_msg
         = "cannot access %<" + js_needle + "%> file descriptors";