libuv: do not require PATH_MAX to be defined

Some platforms (e.g. GNU/Hurd) do not define PATH_MAX.  Add a few other
variants and a fallback constant.  Also use alternatives where possible:

* For readlink(), use lstat() to read the length of the link first.
  If it is not a symlink, report EINVAL before trying to allocate.
  If the size reports as zero, fall back one of the PATH_MAX variants.

* For realpath(), POSIX 2008 allows us to pass a NULL buffer
  to tell it to malloc() internally.

This patch was inspired by downstream patches in Debian packaging
for issues 897061 and 909011.

Issue: #18337

1 files changed, 39 insertions, 8 deletions

diff --git a/Utilities/cmlibuv/src/unix/fs.c b/Utilities/cmlibuv/src/unix/fs.c
index 4545168f29..a6cc6dbd1f 100644
--- a/Utilities/cmlibuv/src/unix/fs.c
+++ b/Utilities/cmlibuv/src/unix/fs.c
@@ -425,19 +425,22 @@ static ssize_t uv__fs_scandir(uv_fs_t* req) {
   return n;
 }
 
+#if defined(_POSIX_PATH_MAX)
+# define UV__FS_PATH_MAX _POSIX_PATH_MAX
+#elif defined(PATH_MAX)
+# define UV__FS_PATH_MAX PATH_MAX
+#else
+# define UV__FS_PATH_MAX_FALLBACK 8192
+# define UV__FS_PATH_MAX UV__FS_PATH_MAX_FALLBACK
+#endif
 
 static ssize_t uv__fs_pathmax_size(const char* path) {
   ssize_t pathmax;
 
   pathmax = pathconf(path, _PC_PATH_MAX);
 
-  if (pathmax == -1) {
-#if defined(PATH_MAX)
-    return PATH_MAX;
-#else
-#error "PATH_MAX undefined in the current platform"
-#endif
-  }
+  if (pathmax == -1)
+    pathmax = UV__FS_PATH_MAX;
 
   return pathmax;
 }
@@ -446,7 +449,28 @@ static ssize_t uv__fs_readlink(uv_fs_t* req) {
   ssize_t len;
   char* buf;
 
+#if defined(UV__FS_PATH_MAX_FALLBACK)
+  /* We may not have a real PATH_MAX.  Read size of link.  */
+  struct stat st;
+  int ret;
+  ret = lstat(req->path, &st);
+  if (ret != 0)
+    return -1;
+  if (!S_ISLNK(st.st_mode)) {
+    errno = EINVAL;
+    return -1;
+  }
+
+  len = st.st_size;
+
+  /* According to readlink(2) lstat can report st_size == 0
+     for some symlinks, such as those in /proc or /sys.  */
+  if (len == 0)
+    len = uv__fs_pathmax_size(req->path);
+#else
   len = uv__fs_pathmax_size(req->path);
+#endif
+
   buf = uv__malloc(len + 1);
 
   if (buf == NULL) {
@@ -473,9 +497,15 @@ static ssize_t uv__fs_readlink(uv_fs_t* req) {
 }
 
 static ssize_t uv__fs_realpath(uv_fs_t* req) {
-  ssize_t len;
   char* buf;
 
+#if defined(_POSIX_VERSION) && _POSIX_VERSION >= 200809L
+  buf = realpath(req->path, NULL);
+  if (buf == NULL)
+    return -1;
+#else
+  ssize_t len;
+
   len = uv__fs_pathmax_size(req->path);
   buf = uv__malloc(len + 1);
 
@@ -488,6 +518,7 @@ static ssize_t uv__fs_realpath(uv_fs_t* req) {
     uv__free(buf);
     return -1;
   }
+#endif
 
   req->ptr = buf;