src: use ftruncate() unconditionally

Systems without it need to provide a custom alternative just like we
have have for Windows. This adds an MSDOS version that fails if trying
to truncate a too large file.

Closes #21109

src/tool_cb_hdr.c

@@ -272,15 +272,9 @@ static size_t save_etag(const char *etag_h, const char *endp,
         /*
          * Truncate regular files to avoid stale etag content.
          */
-#ifdef HAVE_FTRUNCATE
         if(ftruncate(fileno(etag_save->stream), 0))
           return CURL_WRITEFUNC_ERROR;
-#else
-        if(fseek(etag_save->stream, 0, SEEK_SET))
-          return CURL_WRITEFUNC_ERROR;
-#endif
       }
-
       fwrite(etag_h, 1, etag_length, etag_save->stream);
       /* terminate with newline */
       fputc('\n', etag_save->stream);

src/tool_operate.c

@@ -558,7 +558,7 @@ static CURLcode retrycheck(struct OperationConfig *config,
         fflush(outs->stream);
         notef("Throwing away %" CURL_FORMAT_CURL_OFF_T " bytes", outs->bytes);
         /* truncate file at the position where we started appending */
-#if defined(HAVE_FTRUNCATE) && !defined(__DJGPP__) && !defined(__AMIGA__)
+
         if(ftruncate(fileno(outs->stream), outs->init)) {
           /* when truncate fails, we cannot append as then we
              create something strange, bail out */
@@ -568,11 +568,7 @@ static CURLcode retrycheck(struct OperationConfig *config,
         /* now seek to the end of the file, the position where we
            truncated the file in a large file-safe way */
         rc = fseek(outs->stream, 0, SEEK_END);
-#else
-        /* ftruncate is not available, so reposition the file to the location
-           we would have truncated it. */
-        rc = curlx_fseek(outs->stream, outs->init, SEEK_SET);
-#endif
+
         if(rc) {
           errorf("Failed seeking to end of file");
           return CURLE_WRITE_ERROR;

src/tool_setup.h

@@ -108,6 +108,14 @@ int tool_ftruncate64(int fd, curl_off_t where);
 #endif /* !HAVE_FTRUNCATE */
 #endif /* _WIN32 */
 
+#ifdef __DJGPP__
+int msdos_ftruncate(int fd, curl_off_t where);
+#undef HAVE_FTRUNCATE
+#undef ftruncate /* to be sure */
+#define ftruncate(fd, where) msdos_ftruncate(fd, where)
+#define USE_MSDOS_FTRUNCATE 1
+#endif
+
 #ifdef CURL_CA_EMBED
 extern const unsigned char curl_ca_embed[];
 #endif

src/tool_util.c

@@ -96,6 +96,19 @@ int tool_ftruncate64(int fd, curl_off_t where)
 }
 #endif /* USE_TOOL_FTRUNCATE */
 
+#ifdef USE_MSDOS_FTRUNCATE
+/*
+ * Only supports 'off_t' (signed 32 bit) as file size.
+ */
+int msdos_ftruncate(int fd, curl_off_t where)
+{
+  if(where > INT_MAX)
+    return -1;
+
+  return ftruncate(fd, (off_t) where);
+}
+#endif /* USE_MSDOS_FTRUNCATE */
+
 #ifdef _WIN32
 FILE *tool_execpath(const char *filename, char **pathp)
 {