lib/pager: cleanup and extend API

* clean up function names

* add functions to temporary redirect to the pager and then restore
  original terminal output

Signed-off-by: Karel Zak <kzak@redhat.com>

include/pager.h

@@ -1,6 +1,9 @@
 #ifndef UTIL_LINUX_PAGER
 #define UTIL_LINUX_PAGER
 
-void setup_pager(void);
+void pager_redirect(void);
+
+void pager_open(void);
+void pager_close(void);
 
 #endif

lib/pager.c

@@ -17,11 +17,11 @@
 #include "c.h"
 #include "xalloc.h"
 #include "nls.h"
+#include "ttyutils.h"
+#include "pager.h"
 
 #define NULL_DEVICE	"/dev/null"
 
-void setup_pager(void);
-
 static const char *pager_argv[] = { "sh", "-c", NULL, NULL };
 
 struct child_process {
@@ -30,6 +30,10 @@ struct child_process {
 	int in;
 	int out;
 	int err;
+
+	int org_err;
+	int org_out;
+
 	unsigned no_stdin:1;
 	void (*preexec_cb)(void);
 };
@@ -144,6 +148,9 @@ static void pager_preexec(void)
 
 static void wait_for_pager(void)
 {
+	if (pager_process.pid == 0)
+		return;
+
 	fflush(stdout);
 	fflush(stderr);
 	/* signal EOF to pager */
@@ -158,7 +165,7 @@ static void wait_for_pager_signal(int signo)
 	raise(signo);
 }
 
-void setup_pager(void)
+static void __setup_pager(void)
 {
 	const char *pager = getenv("PAGER");
 
@@ -191,10 +198,50 @@ void setup_pager(void)
 	signal(SIGTERM, wait_for_pager_signal);
 	signal(SIGQUIT, wait_for_pager_signal);
 	signal(SIGPIPE, wait_for_pager_signal);
+}
+
+/* Setup pager and redirects output to the $PAGER. The pager is closed at exit.
+ */
+void pager_redirect(void)
+{
+	if (pager_process.pid)
+		return;		/* already running */
+
+	__setup_pager();
 
 	atexit(wait_for_pager);
 }
 
+/* Setup pager and redirect output, the pager may be closed by pager_close().
+ */
+void pager_open(void)
+{
+	if (pager_process.pid)
+		return;		/* already running */
+
+	pager_process.org_out = dup(STDOUT_FILENO);
+	pager_process.org_err = dup(STDERR_FILENO);
+
+	__setup_pager();
+}
+
+/* Close pager and restore original std{out,err}.
+ */
+void pager_close(void)
+{
+	if (pager_process.pid == 0)
+		return;
+
+	wait_for_pager();
+	dup2(pager_process.org_out, STDOUT_FILENO);
+	dup2(pager_process.org_err, STDERR_FILENO);
+
+	close(pager_process.org_out);
+	close(pager_process.org_err);
+
+	memset(&pager_process, 0, sizeof(pager_process));
+}
+
 #ifdef TEST_PROGRAM
 
 #define MAX 255
@@ -204,7 +251,7 @@ int main(int argc __attribute__ ((__unused__)),
 {
 	int i;
 
-	setup_pager();
+	pager_setup();
 	for (i = 0; i < MAX; i++)
 		printf("%d\n", i);
 	return EXIT_SUCCESS;

sys-utils/dmesg.c

@@ -1418,7 +1418,7 @@ int main(int argc, char *argv[])
 		nopager = 1;
 	ctl.pager = nopager ? 0 : ctl.pager;
 	if (ctl.pager)
-		setup_pager();
+		pager_redirect();
 
 	switch (ctl.action) {
 	case SYSLOG_ACTION_READ_ALL:
@@ -1432,7 +1432,7 @@ int main(int argc, char *argv[])
 			    errx(EXIT_FAILURE, _("--raw can be used together with --level or "
 				 "--facility only when reading messages from /dev/kmsg"));
 		if (ctl.pager)
-			setup_pager();
+			pager_redirect();
 		n = read_buffer(&ctl, &buf);
 		if (n > 0)
 			print_buffer(&ctl, buf, n);