Introduce trivial new pager.c helper infrastructure

This introduces the new function

	void setup_pager(void);

to set up output to be written through a pager applocation.

All in preparation for doing the simple scripts in C.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Makefile

@@ -205,7 +205,7 @@ LIB_OBJS = \
 	quote.o read-cache.o refs.o run-command.o \
 	server-info.o setup.o sha1_file.o sha1_name.o strbuf.o \
 	tag.o tree.o usage.o config.o environment.o ctype.o copy.o \
-	fetch-clone.o revision.o \
+	fetch-clone.o revision.o pager.o \
 	$(DIFF_OBJS)
 
 LIBS = $(LIB_FILE)

cache.h

@@ -352,4 +352,7 @@ extern int copy_fd(int ifd, int ofd);
 extern int receive_unpack_pack(int fd[2], const char *me, int quiet);
 extern int receive_keep_pack(int fd[2], const char *me, int quiet);
 
+/* pager.c */
+extern void setup_pager(void);
+
 #endif /* CACHE_H */

pager.c

@@ -0,0 +1,48 @@
+#include "cache.h"
+
+/*
+ * This is split up from the rest of git so that we might do
+ * something different on Windows, for example.
+ */
+
+static void run_pager(void)
+{
+	const char *prog = getenv("PAGER");
+	if (!prog)
+		prog = "less";
+	setenv("LESS", "-S", 0);
+	execlp(prog, prog, NULL);
+}
+
+void setup_pager(void)
+{
+	pid_t pid;
+	int fd[2];
+
+	if (!isatty(1))
+		return;
+	if (pipe(fd) < 0)
+		return;
+	pid = fork();
+	if (pid < 0) {
+		close(fd[0]);
+		close(fd[1]);
+		return;
+	}
+
+	/* return in the child */
+	if (!pid) {
+		dup2(fd[1], 1);
+		close(fd[0]);
+		close(fd[1]);
+		return;
+	}
+
+	/* The original process turns into the PAGER */
+	dup2(fd[0], 0);
+	close(fd[0]);
+	close(fd[1]);
+
+	run_pager();
+	exit(255);
+}