Support reading package args from stdin

Only occurs if no arguments were provided directly. Arguments can be
separated by any amount of valid whitespace. This allows for piping into
pacman from other programs or from itself, e.g.:

  pacman -Qdtq | pacman -Rs

This is better than using xargs, as xargs will not reconnect stdin to
the terminal. The above operation performed using xargs would require
the --noconfirm flag to be passed to pacman.

Signed-off-by: Dave Reisner <d@falconindy.com>

src/pacman/pacman.c

@@ -26,6 +26,7 @@
 #define PACKAGE_VERSION GIT_VERSION
 #endif
 
+#include <ctype.h> /* isspace */
 #include <stdlib.h> /* atoi */
 #include <stdio.h>
 #include <limits.h>
@@ -1305,6 +1306,38 @@ int main(int argc, char *argv[])
 		cleanup(ret);
 	}
 
+	/* read package arguments from stdin if we have none yet */
+	if(!pm_targets && !isatty(fileno(stdin))) {
+		char line[PATH_MAX];
+		int i = 0;
+		while(i < PATH_MAX && (line[i] = fgetc(stdin)) != EOF) {
+			if(isspace((unsigned char)line[i])) {
+				line[i] = '\0';
+				/* avoid adding zero length arg when multiple spaces separate args */
+				if(i > 0) {
+					pm_targets = alpm_list_add(pm_targets, strdup(line));
+					i = 0;
+				}
+			} else {
+				i++;
+			}
+		}
+		/* check for buffer overflow */
+		if (i >= PATH_MAX) {
+			pm_printf(PM_LOG_ERROR, _("buffer overflow detected in arg parsing\n"));
+			cleanup(EXIT_FAILURE);
+		}
+
+		/* end of stream -- check for data still in line buffer */
+		if(i > 0) {
+			pm_targets = alpm_list_add(pm_targets, strdup(line));
+		}
+		if (!freopen(ctermid(NULL), "r", stdin)) {
+			pm_printf(PM_LOG_ERROR, _("failed to reopen stdin for reading: (%s)\n"),
+					strerror(errno));
+		}
+	}
+
 	/* parse the config file */
 	ret = parseconfig(config->configfile);
 	if(ret != 0) {