Fix execv() failure handling and argv0 usage. (#105)

In some cases:

  - we would use argv[0] as the binary to invoke again in sub-programs
  - execv() would fail with "No such file or directory"

Then our handling of execv() failure was leaving a process and a pipe
behind, leading to a strange process tree and a bad situation.

This patch fixes the code so that we get the realpath for the currently
executed program (using proper API in macOS but a heuristic for other OSes),
and fixes the error handling at the execv() call site too.

src/bin/lib/subcommands.c/runprogram.h

@@ -224,6 +224,10 @@ execute_program(Program *prog)
 			{
 				prog->returnCode = -1;
 				prog->error = errno;
+
+				fprintf(stdout, "%s\n", strerror(errno));
+				fprintf(stderr, "%s\n", strerror(errno));
+				exit(EXIT_CODE_INTERNAL_ERROR);
 			}
 			return;
 		}

src/bin/pg_autoctl/cli_root.h

@@ -14,6 +14,8 @@
 #include "commandline.h"
 
 extern char pg_autoctl_argv0[];
+extern char pg_autoctl_program[];
+extern int logLevel;
 
 extern CommandLine help;
 extern CommandLine version;

src/bin/pg_autoctl/file_utils.c

@@ -426,7 +426,7 @@ unlink_file(const char *filename)
  * then argv[0] (here pg_autoctl_argv0) is just "pg_autoctl".
  */
 bool
-get_program_absolute_path(char *program, int size)
+set_program_absolute_path(char *program, int size)
 {
 #if defined(__APPLE__)
 	int actualSize = _NSGetExecutablePath(program, (uint32_t *) &size);
@@ -438,6 +438,9 @@ get_program_absolute_path(char *program, int size)
 				  "to %d bytes only", actualSize, size);
 		return false;
 	}
+
+	log_debug("Found absolute program: \"%s\"", program);
+
 #else
 	/*
 	 * On Linux and FreeBSD and Solaris, we can find a symbolic link to our

src/bin/pg_autoctl/file_utils.h

@@ -28,6 +28,6 @@ void path_in_same_directory(const char *basePath,
 int search_pathlist(const char *pathlist, const char *filename, char ***result);
 void search_pathlist_destroy_result(char **result);
 bool unlink_file(const char *filename);
-bool get_program_absolute_path(char *program, int size);
+bool set_program_absolute_path(char *program, int size);
 
 #endif /* FILE_UTILS_H */

src/bin/pg_autoctl/main.c

@@ -17,6 +17,7 @@
 #include "keeper_config.h"
 
 char pg_autoctl_argv0[MAXPGPATH];
+char pg_autoctl_program[MAXPGPATH];
 
 /*
  * Main entry point for the binary.
@@ -26,12 +27,6 @@ main(int argc, char **argv)
 {
 	CommandLine command = root;
 
-	/*
-	 * Stash away the ARGV[0] used to run this program, we might need it to
-	 * fill in our systemd service unit configuration file later.
-	 */
-	strlcpy(pg_autoctl_argv0, argv[0], MAXPGPATH);
-
 	/*
 	 * When PG_AUTOCTL_DEBUG is set in the environement, provide the user
 	 * commands available to debug a pg_autoctl instance.
@@ -64,6 +59,25 @@ main(int argc, char **argv)
 	 */
 	log_use_colors(isatty(fileno(stderr)));
 
+	/*
+	 * Stash away the argv[0] used to run this program and compute the realpath
+	 * of the program invoked, which we need at several places including when
+	 * preparing the systemd unit files.
+	 *
+	 * Note that we're using log_debug() in get_program_absolute_path and we
+	 * have not set the log level from the command line option parsing yet. We
+	 * hard-coded LOG_INFO as our log level. For now we won't see the log_debug
+	 * output, but as a developer you could always change the LOG_INFO to
+	 * LOG_DEBUG above and then see the message.
+	 */
+	strlcpy(pg_autoctl_argv0, argv[0], MAXPGPATH);
+
+	if (!set_program_absolute_path(pg_autoctl_program, MAXPGPATH))
+	{
+		/* errors have already been logged */
+		exit(EXIT_CODE_INTERNAL_ERROR);
+	}
+
 	(void) commandline_run(&command, argc, argv);
 
 	return 0;

src/bin/pg_autoctl/systemd_config.c

@@ -77,7 +77,6 @@ void
 systemd_config_init(SystemdServiceConfig *config, const char *pgdata)
 {
 	IniOption systemdOptions[] = SET_INI_OPTIONS_ARRAY(config);
-	char program[MAXPGPATH] = { 0 };
 
 	/* time to setup config->pathnames.systemd */
 	snprintf(config->pathnames.systemd, MAXPGPATH,
@@ -91,13 +90,7 @@ systemd_config_init(SystemdServiceConfig *config, const char *pgdata)
 
 	strlcpy(config->User, config->pgSetup.username, NAMEDATALEN);
 
-	if (!get_program_absolute_path(program, MAXPGPATH))
-	{
-		/* errors have already been logged */
-		exit(EXIT_CODE_INTERNAL_ERROR);
-	}
-
-	snprintf(config->ExecStart, BUFSIZE, "%s run", program);
+	snprintf(config->ExecStart, BUFSIZE, "%s run", pg_autoctl_program);
 
 	if (!ini_validate_options(systemdOptions))
 	{