perf(capture): skip reading the capture file when nothing was written

AUTHORS

@@ -204,6 +204,7 @@ Hamza Mobeen
 Harald Armin Massa
 Harshna
 Henk-Jaap Wagenaar
+Henry Schreiner
 Holger Kohr
 Hugo van Kemenade
 Hui Wang (coldnight)

changelog/14687.improvement.rst

@@ -0,0 +1 @@
+The default fd-level output capturing (``--capture=fd``) now skips reading the capture buffer for tests which produce no output, reducing per-test overhead in large test suites.

src/_pytest/capture.py

@@ -566,6 +566,9 @@ class FDCaptureBinary(FDCaptureBase[bytes]):
 
     def snap(self) -> bytes:
         self._assert_state("snap", ("started", "suspended"))
+        # Avoid seek/read/truncate in the common case of no output.
+        if os.fstat(self.tmpfile.fileno()).st_size == 0:
+            return self.EMPTY_BUFFER
         self.tmpfile.seek(0)
         res = self.tmpfile.buffer.read()
         self.tmpfile.seek(0)
@@ -588,6 +591,9 @@ class FDCapture(FDCaptureBase[str]):
 
     def snap(self) -> str:
         self._assert_state("snap", ("started", "suspended"))
+        # Avoid seek/read/truncate in the common case of no output.
+        if os.fstat(self.tmpfile.fileno()).st_size == 0:
+            return self.EMPTY_BUFFER
         self.tmpfile.seek(0)
         res = self.tmpfile.read()
         self.tmpfile.seek(0)