fix: iteration block hook injection [backport 2.21]

ddtrace/internal/injection.py

@@ -7,6 +7,7 @@
 from typing import Tuple  # noqa:F401
 
 from bytecode import Bytecode
+from bytecode import Instr
 
 from ddtrace.internal.assembly import Assembly
 
@@ -109,6 +110,11 @@ def _inject_hook(code: Bytecode, hook: HookType, lineno: int, arg: Any) -> None:
         raise InvalidLine("Line %d does not exist or is either blank or a comment" % lineno)
 
     for i in locs:
+        if isinstance(instr := code[i], Instr) and instr.name.startswith("END_"):
+            # This is the end of a block, e.g. a for loop. We have already
+            # instrumented the block on entry, so we skip instrumenting the
+            # end as well.
+            continue
         code[i:i] = INJECTION_ASSEMBLY.bind(dict(hook=hook, arg=arg), lineno=lineno)
 
 

releasenotes/notes/fix-iteration-block-injection-af354b7004554dfd.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    dynamic instrumentation: fixed an issue with the instrumentation of the
+    first line of an iteration block (e.g. for loops) that could have caused
+    undefined behavior.

tests/internal/test_injection.py

@@ -253,3 +253,18 @@ def test_finally():
     f()
 
     hook.assert_called_once_with(arg)
+
+
+def test_for_block():
+    def for_loop():
+        a = []
+        for i in range(10):
+            a.append(i)
+        return a
+
+    hook, arg = mock.Mock(), mock.Mock()
+
+    with injected_hook(for_loop, hook, arg, line=for_loop.__code__.co_firstlineno + 2):
+        for_loop()
+
+    hook.assert_called_once_with(arg)