Raise instead of hanging on a truncated frame in Zstd.decompress

decode_one_frame looped until ZSTD_decompressStream returned 0. For a
truncated/incomplete frame libzstd keeps returning a non-zero "need more
input" hint while consuming and producing nothing, so the loop spun
forever. Because ZSTD_decompressStream is called directly (GVL held),
this froze the whole VM at 100% CPU and ignored SIGTERM.

A header-only frame reproduces it:

    Zstd.decompress("\x28\xB5\x2F\xFD")  # hung forever

Detect the no-progress case (no output produced and no input consumed
with a non-zero return) and raise, matching the streaming decompressor
which already stops when the input is exhausted. Add a regression spec.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: Watson1978

ext/zstdruby/zstdruby.c

@@ -51,6 +51,7 @@ static VALUE decode_one_frame(ZSTD_DCtx* dctx, const unsigned char* src, size_t
 
   for (;;) {
     ZSTD_outBuffer o = (ZSTD_outBuffer){ buf, cap, 0 };
+    size_t const in_pos_before = in.pos;
     size_t ret = ZSTD_decompressStream(dctx, &o, &in);
     if (ZSTD_isError(ret)) {
       xfree(buf);
@@ -62,6 +63,13 @@ static VALUE decode_one_frame(ZSTD_DCtx* dctx, const unsigned char* src, size_t
     if (ret == 0) {
       break;
     }
+    /* A non-zero return is a "need more input" hint, not an error, and libzstd's
+       own noForwardProgress guard is bypassed by the early return it takes on a
+       truncated frame header -- so the stall has to be detected here. */
+    if (o.pos == 0 && in.pos == in_pos_before) {
+      xfree(buf);
+      rb_raise(rb_eRuntimeError, "ZSTD_decompressStream failed: truncated or incomplete frame");
+    }
   }
   xfree(buf);
   if (consumed) {

spec/zstd-ruby_spec.rb

@@ -116,6 +116,18 @@ def to_str
       expect { Zstd.decompress(Object.new) }.to raise_error(TypeError)
     end
 
+    it 'should raise (not hang) on a truncated frame' do
+      full = Zstd.compress('a' * 2000)
+      [
+        "\x28\xB5\x2F\xFD".b, # bare zstd magic, no body
+        full.byteslice(0, 5),
+        full.byteslice(0, 6),
+        full.byteslice(0, full.bytesize / 2),
+      ].each do |truncated|
+        expect { Zstd.decompress(truncated) }.to raise_error(RuntimeError)
+      end
+    end
+
     class DummyForDecompress
       def to_str
         Zstd.compress('abc')