[PATCH v2] filter: basic write hooking infrastructure

[Jason at zx2c4.com (Jason A. Donenfeld)]

Filters can now call hook_write and unhook_write if they want to
redirect writing to stdout to a different function. This saves us from
potential file descriptor pipes and other less efficient mechanisms.

We do this instead of replacing the call in html_raw because some places
stdlib's printf functions are used (ui-patch or within git itself),
which has its own internal buffering, which makes it difficult to
interlace our function calls. So, we dlsym libc's write and then
override it in the link stage.

Signed-off-by: Jason A. Donenfeld <Jason at zx2c4.com>
---
 cgit.c   |  2 ++
 cgit.h   |  1 +
 cgit.mk  |  4 +++-
 filter.c | 30 ++++++++++++++++++++++++++++++
 4 files changed, 36 insertions(+), 1 deletion(-)

diff --git a/cgit.c b/cgit.c
index 4f31e58..725fd65 100644
--- a/cgit.c
+++ b/cgit.c
@@ -904,6 +904,8 @@ int main(int argc, const char **argv)
 	const char *path;
 	int err, ttl;
 
+	cgit_init_filters();
+
 	prepare_context(&ctx);
 	cgit_repolist.length = 0;
 	cgit_repolist.count = 0;
diff --git a/cgit.h b/cgit.h
index 893c38f..db34493 100644
--- a/cgit.h
+++ b/cgit.h
@@ -357,6 +357,7 @@ extern void cgit_fprintf_filter(struct cgit_filter *filter, FILE *f, const char
 extern void cgit_exec_filter_init(struct cgit_exec_filter *filter, char *cmd, char **argv);
 extern struct cgit_filter *cgit_new_filter(const char *cmd, filter_type filtertype);
 extern void cgit_cleanup_filters(void);
+extern void cgit_init_filters(void);
 
 extern void cgit_prepare_repo_env(struct cgit_repo * repo);
 
diff --git a/cgit.mk b/cgit.mk
index 19a76e7..9d6dea8 100644
--- a/cgit.mk
+++ b/cgit.mk
@@ -61,6 +61,8 @@ $(CGIT_VERSION_OBJS): $(CGIT_PREFIX)VERSION
 $(CGIT_VERSION_OBJS): EXTRA_CPPFLAGS = \
 	-DCGIT_VERSION='"$(CGIT_VERSION)"'
 
+CGIT_LIBS += -ldl
+
 
 # Git handles dependencies using ":=" so dependencies in CGIT_OBJ are not
 # handled by that and we must handle them ourselves.
@@ -88,4 +90,4 @@ $(CGIT_OBJS): %.o: %.c GIT-CFLAGS $(CGIT_PREFIX)CGIT-CFLAGS $(missing_dep_dirs)
 	$(QUIET_CC)$(CC) -o $*.o -c $(dep_args) $(ALL_CFLAGS) $(EXTRA_CPPFLAGS) $(CGIT_CFLAGS) $<
 
 $(CGIT_PREFIX)cgit: $(CGIT_OBJS) GIT-LDFLAGS $(GITLIBS)
-	$(QUIET_LINK)$(CC) $(ALL_CFLAGS) -o $@ $(ALL_LDFLAGS) $(filter %.o,$^) $(LIBS)
+	$(QUIET_LINK)$(CC) $(ALL_CFLAGS) -o $@ $(ALL_LDFLAGS) $(filter %.o,$^) $(LIBS) $(CGIT_LIBS)
diff --git a/filter.c b/filter.c
index 86c1d5d..8990575 100644
--- a/filter.c
+++ b/filter.c
@@ -12,6 +12,10 @@
 #include <unistd.h>
 #include <string.h>
 #include <stdlib.h>
+#include <dlfcn.h>
+
+static ssize_t (*libc_write)(int fd, const void *buf, size_t count);
+static ssize_t (*filter_write)(const void *buf, size_t count) = NULL;
 
 static int open_exec_filter(struct cgit_filter *base, va_list ap)
 {
@@ -192,3 +196,29 @@ void cgit_cleanup_filters(void)
 		reap_filter(cgit_repolist.repos[i].source_filter);
 	}
 }
+
+void cgit_init_filters(void)
+{
+	libc_write = dlsym(RTLD_NEXT, "write");
+	if (!libc_write)
+		die("Could not locate libc's write function");
+}
+
+ssize_t write(int fd, const void *buf, size_t count)
+{
+	if (fd != STDOUT_FILENO || !filter_write)
+		return libc_write(fd, buf, count);
+	return filter_write(buf, count);
+}
+
+static inline void hook_write(ssize_t (*new_write)(const void *buf, size_t count))
+{
+	/* We want to avoid buggy nested patterns. */
+	assert(filter_write == NULL);
+	filter_write = new_write;
+}
+static inline void unhook_write()
+{
+	assert(filter_write != NULL);
+	filter_write = NULL;
+}
-- 
1.8.5.2