xargs: optional support for -P NUM. Closes 9511

Based on patch by Johannes Schindelin <johannes.schindelin@gmx.de>

function                                             old     new   delta
xargs_exec                                             -     294    +294
packed_usage                                       31757   31772     +15
xargs_main                                           787     719     -68
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 1/1 up/down: 309/-68)           Total: 241 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

findutils/xargs.c

@@ -59,6 +59,11 @@
 //config:	depends on XARGS
 //config:	help
 //config:	Support -I STR and -i[STR] options.
+//config:
+//config:config FEATURE_XARGS_SUPPORT_PARALLEL
+//config:	bool "Enable -P N: processes to run in parallel"
+//config:	default y
+//config:	depends on XARGS
 
 //applet:IF_XARGS(APPLET_NOEXEC(xargs, xargs, BB_DIR_USR_BIN, BB_SUID_DROP, xargs))
 
@@ -99,38 +104,121 @@ struct globals {
 #endif
 	const char *eof_str;
 	int idx;
+#if ENABLE_FEATURE_XARGS_SUPPORT_PARALLEL
+	int running_procs;
+	int max_procs;
+#endif
+	smalluint xargs_exitcode;
 } FIX_ALIASING;
 #define G (*(struct globals*)bb_common_bufsiz1)
 #define INIT_G() do { \
 	setup_common_bufsiz(); \
 	G.eof_str = NULL; /* need to clear by hand because we are NOEXEC applet */ \
 	G.idx = 0; \
+	IF_FEATURE_XARGS_SUPPORT_PARALLEL(G.running_procs = 0;) \
+	IF_FEATURE_XARGS_SUPPORT_PARALLEL(G.max_procs = 1;) \
+	G.xargs_exitcode = 0; \
 	IF_FEATURE_XARGS_SUPPORT_REPL_STR(G.repl_str = "{}";) \
 	IF_FEATURE_XARGS_SUPPORT_REPL_STR(G.eol_ch = '\n';) \
 } while (0)
 
 
+/*
+ * Returns 0 if xargs should continue (but may set G.xargs_exitcode to 123).
+ * Else sets G.xargs_exitcode to error code and returns nonzero.
+ *
+ * If G.max_procs == 0, performs final waitpid() loop for all children.
+ */
 static int xargs_exec(void)
 {
 	int status;
 
+#if !ENABLE_FEATURE_XARGS_SUPPORT_PARALLEL
 	status = spawn_and_wait(G.args);
+#else
+	if (G.max_procs == 1) {
+		status = spawn_and_wait(G.args);
+	} else {
+		pid_t pid;
+		int wstat;
+ again:
+		if (G.running_procs >= G.max_procs)
+			pid = safe_waitpid(-1, &wstat, 0);
+		else
+			pid = wait_any_nohang(&wstat);
+		if (pid > 0) {
+			/* We may have children we don't know about:
+			 * sh -c 'sleep 1 & exec xargs ...'
+			 * Do not make G.running_procs go negative.
+			 */
+			if (G.running_procs != 0)
+				G.running_procs--;
+			status = WIFSIGNALED(wstat)
+				? 0x180 + WTERMSIG(wstat)
+				: WEXITSTATUS(wstat);
+			if (status > 0 && status < 255) {
+				/* See below why 123 does not abort */
+				G.xargs_exitcode = 123;
+				status = 0;
+			}
+			if (status == 0)
+				goto again; /* maybe we have more children? */
+			/* else: "bad" status, will bail out */
+		} else if (G.max_procs != 0) {
+			/* Not in final waitpid() loop,
+			 * and G.running_procs < G.max_procs: start more procs
+			 */
+			status = spawn(G.args);
+			/* here "status" actually holds pid, or -1 */
+			if (status > 0) {
+				G.running_procs++;
+				status = 0;
+			}
+			/* else: status == -1 (failed to fork or exec) */
+		} else {
+			/* final waitpid() loop: must be ECHILD "no more children" */
+			status = 0;
+		}
+	}
+#endif
+	/* Manpage:
+	 * """xargs exits with the following status:
+	 * 0 if it succeeds
+	 * 123 if any invocation of the command exited with status 1-125
+	 * 124 if the command exited with status 255
+	 *     ("""If any invocation of the command exits with a status of 255,
+	 *     xargs will stop immediately without reading any further input.
+	 *     An error message is issued on stderr when this happens.""")
+	 * 125 if the command is killed by a signal
+	 * 126 if the command cannot be run
+	 * 127 if the command is not found
+	 * 1 if some other error occurred."""
+	 */
 	if (status < 0) {
 		bb_simple_perror_msg(G.args[0]);
-		return errno == ENOENT ? 127 : 126;
+		status = (errno == ENOENT) ? 127 : 126;
 	}
-	if (status == 255) {
+	else if (status >= 0x180) {
-		bb_error_msg("%s: exited with status 255; aborting", G.args[0]);
-		return 124;
-	}
-	if (status >= 0x180) {
 		bb_error_msg("'%s' terminated by signal %d",
 			G.args[0], status - 0x180);
-		return 125;
+		status = 125;
 	}
-	if (status)
+	else if (status != 0) {
-		return 123;
+		if (status == 255) {
-	return 0;
+			bb_error_msg("%s: exited with status 255; aborting", G.args[0]);
+			return 124;
+		}
+		/* "123 if any invocation of the command exited with status 1-125"
+		 * This implies that nonzero exit code is remembered,
+		 * but does not cause xargs to stop: we return 0.
+		 */
+		G.xargs_exitcode = 123;
+		status = 0;
+	}
+
+	if (status != 0)
+		G.xargs_exitcode = status;
+	return status;
 }
 
 /* In POSIX/C locale isspace is only these chars: "\t\n\v\f\r" and space.
@@ -436,6 +524,9 @@ static int xargs_ask_confirmation(void)
 //usage:	IF_FEATURE_XARGS_SUPPORT_REPL_STR(
 //usage:     "\n	-I STR	Replace STR within PROG ARGS with input line"
 //usage:	)
+//usage:	IF_FEATURE_XARGS_SUPPORT_PARALLEL(
+//usage:     "\n	-P N	Run up to N PROGs in parallel"
+//usage:	)
 //usage:	IF_FEATURE_XARGS_SUPPORT_TERMOPT(
 //usage:     "\n	-x	Exit if size is exceeded"
 //usage:	)
@@ -473,14 +564,14 @@ enum {
 	IF_FEATURE_XARGS_SUPPORT_CONFIRMATION("p") \
 	IF_FEATURE_XARGS_SUPPORT_TERMOPT(     "x") \
 	IF_FEATURE_XARGS_SUPPORT_ZERO_TERM(   "0") \
-	IF_FEATURE_XARGS_SUPPORT_REPL_STR(    "I:i::")
+	IF_FEATURE_XARGS_SUPPORT_REPL_STR(    "I:i::") \
+	IF_FEATURE_XARGS_SUPPORT_PARALLEL(    "P:+")
 
 int xargs_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int xargs_main(int argc UNUSED_PARAM, char **argv)
 {
 	int initial_idx;
 	int i;
-	int child_error = 0;
 	char *max_args;
 	char *max_chars;
 	char *buf;
@@ -500,8 +591,14 @@ int xargs_main(int argc UNUSED_PARAM, char **argv)
 		"no-run-if-empty\0" No_argument "r",
 		&max_args, &max_chars, &G.eof_str, &G.eof_str
 		IF_FEATURE_XARGS_SUPPORT_REPL_STR(, &G.repl_str, &G.repl_str)
+		IF_FEATURE_XARGS_SUPPORT_PARALLEL(, &G.max_procs)
 	);
 
+#if ENABLE_FEATURE_XARGS_SUPPORT_PARALLEL
+	if (G.max_procs <= 0) /* -P0 means "run lots of them" */
+		G.max_procs = 100; /* let's not go crazy high */
+#endif
+
 	/* -E ""? You may wonder why not just omit -E?
 	 * This is used for portability:
 	 * old xargs was using "_" as default for -E / -e */
@@ -620,11 +717,8 @@ int xargs_main(int argc UNUSED_PARAM, char **argv)
 		}
 
 		if (!(opt & OPT_INTERACTIVE) || xargs_ask_confirmation()) {
-			child_error = xargs_exec();
+			if (xargs_exec() != 0)
-		}
+				break; /* G.xargs_exitcode is set by xargs_exec() */
-
-		if (child_error > 0 && child_error != 123) {
-			break;
 		}
 
 		overlapping_strcpy(buf, rem);
@@ -635,7 +729,12 @@ int xargs_main(int argc UNUSED_PARAM, char **argv)
 		free(buf);
 	}
 
-	return child_error;
+#if ENABLE_FEATURE_XARGS_SUPPORT_PARALLEL
+	G.max_procs = 0;
+	xargs_exec(); /* final waitpid() loop */
+#endif
+
+	return G.xargs_exitcode;
 }