fix(static): skip magic-number pread under directio (O_DIRECT EINVAL)

static/ngx_http_zstd_static_module.c

@@ -250,7 +250,17 @@ ngx_http_zstd_static_handler(ngx_http_request_t *r)
      * fall back to a read+lseek pair that would mutate the shared fd
      * offset. Every modern POSIX target has it; this guard is
      * essentially a build-time tripwire.
+     *
+     * Skipped when the file was opened with O_DIRECT (of.is_directio, set by
+     * ngx_open_cached_file when "directio <size>" is configured and the file
+     * meets the threshold). An O_DIRECT read requires the buffer, offset and
+     * length all aligned to the device block size; this deliberately tiny,
+     * unaligned 4-byte pread would fail with EINVAL on every request, wrongly
+     * declining the .zst and spamming NGX_LOG_CRIT. The probe is only a
+     * best-effort corruption guard, so forgoing it under directio is the
+     * correct trade — the file is still served, just without the sanity check.
      */
+    if (!of.is_directio) {
     if (of.size < 4) {
         ngx_log_error(NGX_LOG_ERR, log, 0,
                       "zstd static: \"%s\" too small to be a zstd frame "
@@ -289,6 +299,11 @@ ngx_http_zstd_static_handler(ngx_http_request_t *r)
             return NGX_DECLINED;
         }
     }
+    } else {
+        ngx_log_debug1(NGX_LOG_DEBUG_HTTP, log, 0,
+                       "zstd static: skipping magic probe on directio "
+                       "file \"%s\" (O_DIRECT alignment)", path.data);
+    }
 #endif /* NGX_HAVE_PREAD */
 
 #endif

t/01-static.t

@@ -570,3 +570,33 @@ Accept-Encoding: gzip
 Vary: Accept-Encoding
 --- no_error_log
 [error]
+
+
+
+=== TEST 26: zstd_static serves a .zst under directio without a failed pread
+# Regression for the O_DIRECT magic-probe bug. With "directio" active the
+# open_file_cache opens the .zst with O_DIRECT; the 4-byte, unaligned
+# magic pread() then fails EINVAL, wrongly declining every .zst above the
+# threshold and spamming NGX_LOG_CRIT. The fix skips the probe when
+# of.is_directio. Assert the precompressed frame is still served (200 +
+# Content-Encoding: zstd) and no CRIT/[error] appears.
+#
+# NOTE: O_DIRECT support is filesystem-dependent (tmpfs may not honour it),
+# so this deterministically catches the "declined + CRIT" regression only
+# where O_DIRECT actually engages; everywhere else it still asserts correct
+# serving. directio 1 = every file >= 1 byte is eligible (test.zst is 3717B).
+--- config
+    location /test {
+        zstd_static on;
+        directio 1;
+        root ../../t/suite;
+    }
+--- request
+GET /test
+--- more_headers
+Accept-Encoding: zstd
+--- response_headers
+Content-Encoding: zstd
+--- error_code: 200
+--- no_error_log
+[error]