[PATCHv2 2/4] run_command: handle missing command errors more gracefully

From: Jeff King <peff@peff.net>
Date: Jan 28, 2009, 07:35 UTC
Message-ID: <20090128073533.GB31884@coredump.intra.peff.net>
In-Reply-To: <20090128073059.GD19165@coredump.intra.peff.net>

When run_command was asked to run a non-existant command, its behavior varied depending on the platform:

  - on POSIX systems, we would fork, and then after the
    execvp call failed, we could call die(), which prints a
    message to stderr and exits with code 128.

  - on Windows, we do a PATH lookup, realize the program
    isn't there, and then return ERR_RUN_COMMAND_FORK

The goal of this patch is to make it clear to callers that the specific error was a missing command. To do this, we will return the error code ERR_RUN_COMMAND_EXEC, which is already defined in run-command.h, checked for in several places, but never actually gets set.

The new behavior is:

  - on POSIX systems, we exit the forked process with code
    127 (the same as the shell uses to report missing
    commands). The parent process recognizes this code and
    returns an EXEC error. The stderr message is silenced,
    since the caller may be speculatively trying to run a
    command. Instead, we use trace_printf so that somebody
    interested in debugging can see the error that occured.

  - on Windows, we check errno, which is already set
    correctly by mingw_spawnvpe, and report an EXEC error
    instead of a FORK error

Thus it is safe to speculatively run a command:

  int r = run_command_v_opt(argv, 0);
  if (r == -ERR_RUN_COMMAND_EXEC)
	  /* oops, it wasn't found; try something else */
  else
	  /* we failed for some other reason, error is in r */

Signed-off-by: Jeff King <peff@peff.net>
---
Incorporates JSixt's fix to retain errno across system calls.

 run-command.c |   18 ++++++++++++++----
 1 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/run-command.c b/run-command.c
index db9ce59..b05c734 100644
--- a/run-command.c
+++ b/run-command.c
@@ -118,7 +118,9 @@ int start_command(struct child_process *cmd)
 		} else {
 			execvp(cmd->argv[0], (char *const*) cmd->argv);
 		}
-		die("exec %s failed.", cmd->argv[0]);
+		trace_printf("trace: exec '%s' failed: %s\n", cmd->argv[0],
+				strerror(errno));
+		exit(127);
 	}
 #else
 	int s0 = -1, s1 = -1, s2 = -1;	/* backups of stdin, stdout, stderr */
@@ -187,6 +189,7 @@ int start_command(struct child_process *cmd)
 #endif
 
 	if (cmd->pid < 0) {
+		int err = errno;
 		if (need_in)
 			close_pair(fdin);
 		else if (cmd->in)
@@ -197,7 +200,9 @@ int start_command(struct child_process *cmd)
 			close(cmd->out);
 		if (need_err)
 			close_pair(fderr);
-		return -ERR_RUN_COMMAND_FORK;
+		return err == ENOENT ?
+			-ERR_RUN_COMMAND_EXEC :
+			-ERR_RUN_COMMAND_FORK;
 	}
 
 	if (need_in)
@@ -236,9 +241,14 @@ static int wait_or_whine(pid_t pid)
 		if (!WIFEXITED(status))
 			return -ERR_RUN_COMMAND_WAITPID_NOEXIT;
 		code = WEXITSTATUS(status);
-		if (code)
+		switch (code) {
+		case 127:
+			return -ERR_RUN_COMMAND_EXEC;
+		case 0:
+			return 0;
+		default:
 			return -code;
-		return 0;
+		}
 	}
 }

-- 
1.6.1.1.367.g30b36