chg: fallback to original hg if stdio fds are missing

If stdio fds are missing (ex. fd 0 is not present), chg might open
fds that take the numbers 0, and attachio would send the wrong fds
to the client, which might cause unwanted behaviors. Avoid that by
detecting the missing fds and falling back to the original hg.

Differential Revision: https://phab.mercurial-scm.org/D9058

contrib/chg/chg.c

@@ -373,8 +373,15 b' static int runinstructions(struct cmdser'
 }
 
 /*
- * Test whether the command is unsupported or not. This is not designed to
- * cover all cases. But it's fast, does not depend on the server.
+ * Test whether the command and the environment is unsupported or not.
+ *
+ * If any of the stdio file descriptors are not present (rare, but some tools
+ * might spawn new processes without stdio instead of redirecting them to the
+ * null device), then mark it as not supported because attachio won't work
+ * correctly.
+ *
+ * The command list is not designed to cover all cases. But it's fast, and does
+ * not depend on the server.
  */
 static int isunsupported(int argc, const char *argv[])
 {
@@ -384,6 +391,13 b' static int isunsupported(int argc, const'
 	};
 	unsigned int state = 0;
 	int i;
+	/* use fcntl to test missing stdio fds */
+	if (fcntl(STDIN_FILENO, F_GETFD) == -1 ||
+	    fcntl(STDOUT_FILENO, F_GETFD) == -1 ||
+	    fcntl(STDERR_FILENO, F_GETFD) == -1) {
+		debugmsg("stdio fds are missing");
+		return 1;
+	}
 	for (i = 0; i < argc; ++i) {
 		if (strcmp(argv[i], "--") == 0)
 			break;

tests/test-chg.t

@@ -197,6 +197,14 b' in this test.'
 
   $ cd ..
 
+missing stdio
+-------------
+
+  $ CHGDEBUG=1 chg version -q 0<&-
+  chg: debug: * stdio fds are missing (glob)
+  chg: debug: * execute original hg (glob)
+  Mercurial Distributed SCM * (glob)
+
 server lifecycle
 ----------------