Fix reading special files that lie about their size

source/file.c

@@ -62,25 +62,43 @@ int aws_byte_buf_init_from_file(struct aws_byte_buf *out_buf, struct aws_allocat
         goto error;
     }
 
-    size_t allocation_size = (size_t)len64 + 1;
-    aws_byte_buf_init(out_buf, alloc, allocation_size);
-
-    /* Ensure compatibility with null-terminated APIs, but don't consider
-     * the null terminator part of the length of the payload */
-    out_buf->len = out_buf->capacity - 1;
-    out_buf->buffer[out_buf->len] = 0;
-
-    size_t read = fread(out_buf->buffer, 1, out_buf->len, fp);
-    if (read < out_buf->len) {
-        int errno_value = ferror(fp) ? errno : 0; /* Always cache errno before potential side-effect */
-        aws_translate_and_raise_io_error_or(errno_value, AWS_ERROR_FILE_READ_FAILURE);
-        AWS_LOGF_ERROR(
-            AWS_LS_COMMON_IO,
-            "static: Failed reading file:'%s' errno:%d aws-error:%s",
-            filename,
-            errno_value,
-            aws_error_name(aws_last_error()));
-        goto error;
+    /* For "special" files, size may be a lie. For example, on Amazon Linux 2:
+     * /proc/net/tcp: says 0, but it's larger if you read it.
+     * /sys/devices/virtual/dmi/id/product_name: says 4096, but it's smaller if you read it.
+     *
+     * So we'll use file-size as a hint, but read() in a loop until we hit EOF,
+     * resizing the buffer if necessary as we go.
+     *
+     * If file-size == 0, add a bit of capacity to start.
+     * If file-size != 0, add 1 more to capacity, in case we need to do a 2nd read to learn it's EOF. */
+    size_t initial_capacity = len64 == 0 ? 128 : (size_t)len64 + 1;
+    aws_byte_buf_init(out_buf, alloc, initial_capacity);
+    while (true) {
+        /* Add more capacity if necessary */
+        if (out_buf->len == out_buf->capacity) {
+            size_t additional_capacity = aws_min_size(4096, aws_mul_size_saturating(out_buf->capacity, 2));
+            aws_byte_buf_reserve_relative(out_buf, additional_capacity);
+        }
+
+        size_t space_available = out_buf->capacity - out_buf->len;
+        size_t read = fread(out_buf->buffer + out_buf->len, 1, space_available, fp);
+        out_buf->len += read;
+
+        if (feof(fp)) {
+            break;
+        }
+
+        if (read == 0) {
+            int errno_value = ferror(fp) ? errno : 0; /* Always cache errno before potential side-effect */
+            aws_translate_and_raise_io_error_or(errno_value, AWS_ERROR_FILE_READ_FAILURE);
+            AWS_LOGF_ERROR(
+                AWS_LS_COMMON_IO,
+                "static: Failed reading file:'%s' errno:%d aws-error:%s",
+                filename,
+                errno_value,
+                aws_error_name(aws_last_error()));
+            goto error;
+        }
     }
 
     fclose(fp);