xmlIO: Fix reading from non-regular files like pipes

Commit 7e14c05 removed unnecessary copying of uncompressed input
through zlib or xzlib. This broke input from non-regular files like
pipes which can't be reopened. Try to detect such files by checking
whether they're seekable and always pipe them through zlib or xzlib.

Also remove seemingly unnecessary calls to gzread and gzrewind to
support unseekable files.

Fixes https://gitlab.gnome.org/GNOME/libxslt/-/issues/124.

Author: nwellnhof

xmlIO.c

@@ -1210,7 +1210,13 @@ xmlInputDefaultOpen(xmlParserInputBufferPtr buf, const char *filename) {
         if (xzStream == NULL) {
             close(fd);
         } else {
-            if (__libxml2_xzcompressed(xzStream) > 0) {
+            /*
+             * Non-regular files like pipes can't be reopened.
+             * If a file isn't seekable, we pipe uncompressed
+             * input through xzlib.
+             */
+            if ((lseek(fd, 0, SEEK_CUR) < 0) ||
+                (__libxml2_xzcompressed(xzStream) > 0)) {
                 buf->context = xzStream;
                 buf->readcallback = xmlXzfileRead;
                 buf->closecallback = xmlXzfileClose;
@@ -1237,12 +1243,13 @@ xmlInputDefaultOpen(xmlParserInputBufferPtr buf, const char *filename) {
         if (gzStream == NULL) {
             close(fd);
         } else {
-            char buff4[4];
-
-            if ((gzread(gzStream, buff4, 4) > 0) &&
+            /*
+             * Non-regular files like pipes can't be reopened.
+             * If a file isn't seekable, we pipe uncompressed
+             * input through zlib.
+             */
+            if ((lseek(fd, 0, SEEK_CUR) < 0) ||
                 (gzdirect(gzStream) == 0)) {
-                gzrewind(gzStream);
-
                 buf->context = gzStream;
                 buf->readcallback = xmlGzfileRead;
                 buf->closecallback = xmlGzfileClose;