Support pipe and friends on hb_blob_create_from_file (#1061)

With this hb-view/hb-shape support pipes and possibly socket and named pipe
also, anything fails just on mmap.

We can later do the same for Windows also.

This however reveals two issues, the fact most of our bots don't have HAVE_MMAP and using
this instead the other fread/fopen reader can make failure on CI. I should look at them separately
this change however is very low risk I believe.

src/hb-blob.cc

@@ -499,6 +499,36 @@ hb_blob_t::try_make_writable (void)
 # define MAP_NORESERVE 0
 #endif
 
+static hb_blob_t *
+_fread_to_end (FILE *file)
+{
+  int allocated = BUFSIZ * 16;
+  char *blob = (char *) malloc (allocated);
+  if (blob == nullptr) return hb_blob_get_empty ();
+
+  int len = 0;
+  while (!feof (file))
+  {
+    if (allocated - len < BUFSIZ)
+    {
+      allocated *= 2;
+      char *new_blob = (char *) realloc (blob, allocated);
+      if (unlikely (!new_blob)) goto fail;
+      blob = new_blob;
+    }
+
+    len += fread (blob + len, 1, allocated - len, file);
+    if (ferror (file)) goto fail;
+  }
+
+  return hb_blob_create (blob, len, HB_MEMORY_MODE_WRITABLE, blob,
+			 (hb_destroy_func_t) free);
+
+fail:
+  free (blob);
+  return hb_blob_get_empty ();
+}
+
 struct hb_mapped_file_t
 {
   char *contents;
@@ -557,7 +587,19 @@ hb_blob_create_from_file (const char *file_name)
 				  writable ? PROT_READ|PROT_WRITE : PROT_READ,
 				  MAP_PRIVATE | MAP_NORESERVE, fd, 0);
 
-  if (unlikely (file->contents == MAP_FAILED)) goto fail;
+  if (unlikely (file->contents == MAP_FAILED))
+  {
+    free (file);
+    FILE *f = fdopen (fd, "rb");
+    if (unlikely (f == nullptr))
+    {
+      CLOSE (fd);
+      return hb_blob_get_empty ();
+    }
+    hb_blob_t *blob = _fread_to_end (f);
+    fclose (f);
+    return blob;
+  }
 
 #elif defined(_WIN32) || defined(__CYGWIN__)
   HANDLE fd = CreateFile (file_name,