[musl] [PATCH v1] nftw: support FTW_ACTIONRETVAL extension

[Tony Ambardar <tony.ambardar@gmail.com>]

Add the FTW_ACTIONRETVAL mode for walking file trees, a commonly used glibc
extension. This updates the existing musl nftw/ftw code to support the new
mode and include options e.g. to prune trees and skip tree siblings.

This patch is based on one originally submitted to musl's mailing list [1],
which was reviewed with interest but received no follow-up. It has since
been cleaned up, and tested on OpenWRT for building Linux's 'bpftool'
utility (the primary tool for BPF introspection and manipulation).

Including this extension should also further reduce any need for fts(3)
support.

[1] https://www.openwall.com/lists/musl/2018/12/16/1

Signed-off-by: Tony Ambardar <Tony.Ambardar@gmail.com>
---
 include/ftw.h   |  8 ++++++++
 src/misc/nftw.c | 35 ++++++++++++++++++++++++++++++-----
 2 files changed, 38 insertions(+), 5 deletions(-)

diff --git a/include/ftw.h b/include/ftw.h
index b15c062a..ce85deac 100644
--- a/include/ftw.h
+++ b/include/ftw.h
@@ -21,6 +21,14 @@ extern "C" {
 #define FTW_CHDIR 4
 #define FTW_DEPTH 8
 
+#ifdef _GNU_SOURCE
+#define FTW_ACTIONRETVAL 16
+#define FTW_CONTINUE 0
+#define FTW_STOP 1
+#define FTW_SKIP_SUBTREE 2
+#define FTW_SKIP_SIBLINGS 3
+#endif
+
 struct FTW {
 	int base;
 	int level;
diff --git a/src/misc/nftw.c b/src/misc/nftw.c
index 8dcff7fe..0bb7b601 100644
--- a/src/misc/nftw.c
+++ b/src/misc/nftw.c
@@ -1,3 +1,4 @@
+#define _GNU_SOURCE
 #include <ftw.h>
 #include <dirent.h>
 #include <fcntl.h>
@@ -72,8 +73,20 @@ static int do_nftw(char *path, int (*fn)(const char *, const struct stat *, int,
 		if (!fd_limit) close(dfd);
 	}
 
-	if (!(flags & FTW_DEPTH) && (r=fn(path, &st, type, &lev)))
-		return r;
+	if (!(flags & FTW_DEPTH) && (r=fn(path, &st, type, &lev))) {
+		if (flags & FTW_ACTIONRETVAL)
+			switch (r) {
+			case FTW_SKIP_SUBTREE:
+				h = NULL;
+			case FTW_CONTINUE:
+				break;
+			case FTW_SKIP_SIBLINGS:
+			case FTW_STOP:
+				return r;
+			}
+		else
+			return r;
+	}
 
 	for (; h; h = h->chain)
 		if (h->dev == st.st_dev && h->ino == st.st_ino)
@@ -101,7 +114,10 @@ static int do_nftw(char *path, int (*fn)(const char *, const struct stat *, int,
 				strcpy(path+j+1, de->d_name);
 				if ((r=do_nftw(path, fn, fd_limit-1, flags, &new))) {
 					closedir(d);
-					return r;
+					if ((flags & FTW_ACTIONRETVAL) && r == FTW_SKIP_SIBLINGS)
+						break;
+					else
+						return r;
 				}
 			}
 			closedir(d);
@@ -112,8 +128,16 @@ static int do_nftw(char *path, int (*fn)(const char *, const struct stat *, int,
 	}
 
 	path[l] = 0;
-	if ((flags & FTW_DEPTH) && (r=fn(path, &st, type, &lev)))
-		return r;
+	if ((flags & FTW_DEPTH) && (r=fn(path, &st, type, &lev))) {
+		if (flags & FTW_ACTIONRETVAL)
+			switch (r) {
+				case FTW_SKIP_SIBLINGS:
+				case FTW_STOP:
+					return r;
+			}
+		else
+			return r;
+	}
 
 	return 0;
 }
@@ -139,4 +163,5 @@ int nftw(const char *path, int (*fn)(const char *, const struct stat *, int, str
 	return r;
 }
 
+#undef nftw64
 weak_alias(nftw, nftw64);
-- 
2.25.1