ref: Store tracked spans on start not finish

[rhcarvalho]

This matches the JS implementation, and without it we cannot use the span recorder of a span to find its parent transaction (required to implement #734).

[rhcarvalho]

It turns out that the test failures are due to an off-by-one difference in interpretation of what the max_spans config means.

max_spans = 3 in JS means transaction + 3 child spans.
max_spans = 3 in Python means transaction + 2 child spans.

Going to preserve the inconsistent behavior for now, with a note, since it is not part of the intended work in this PR.

[rhcarvalho]

Changes to get tests passing:

diff --git a/sentry_sdk/tracing.py b/sentry_sdk/tracing.py
index 9f0e197..5e9ae8a 100644
--- a/sentry_sdk/tracing.py
+++ b/sentry_sdk/tracing.py
@@ -73,7 +73,12 @@ class _SpanRecorder(object):
 
     def __init__(self, maxlen):
         # type: (int) -> None
-        self.maxlen = maxlen
+        # FIXME: this is `maxlen - 1` only to preserve historical behavior
+        # enforced by tests.
+        # Either this should be changed to `maxlen` or the JS SDK implementation
+        # should be changed to match a consistent interpretation of what maxlen
+        # limits: either transaction+spans or only child spans.
+        self.maxlen = maxlen - 1
         self.spans = []  # type: List[Span]
 
     def add(self, span):
diff --git a/tests/integrations/stdlib/test_subprocess.py b/tests/integrations/stdlib/test_subprocess.py
index ee6e7c8..e2ae005 100644
--- a/tests/integrations/stdlib/test_subprocess.py
+++ b/tests/integrations/stdlib/test_subprocess.py
@@ -140,13 +140,15 @@ def test_subprocess_basic(
 
     (
         subprocess_init_span,
-        subprocess_wait_span,
         subprocess_communicate_span,
+        subprocess_wait_span,
     ) = transaction_event["spans"]
 
-    assert subprocess_init_span["op"] == "subprocess"
-    assert subprocess_communicate_span["op"] == "subprocess.communicate"
-    assert subprocess_wait_span["op"] == "subprocess.wait"
+    assert (
+        subprocess_init_span["op"],
+        subprocess_communicate_span["op"],
+        subprocess_wait_span["op"],
+    ) == ("subprocess", "subprocess.communicate", "subprocess.wait")
 
     # span hierarchy
     assert (

Plus updated commit message.

[rhcarvalho]

And hopefully last test fix accounting for the change in order of spans.

diff --git a/tests/integrations/django/test_basic.py b/tests/integrations/django/test_basic.py
index b3a08f5..3c26b42 100644
--- a/tests/integrations/django/test_basic.py
+++ b/tests/integrations/django/test_basic.py
@@ -518,10 +518,10 @@ def test_middleware_spans(sentry_init, client, capture_events):
 
     if DJANGO_VERSION >= (1, 10):
         reference_value = [
-            "tests.integrations.django.myapp.settings.TestFunctionMiddleware.__call__",
-            "tests.integrations.django.myapp.settings.TestMiddleware.__call__",
-            "django.contrib.auth.middleware.AuthenticationMiddleware.__call__",
             "django.contrib.sessions.middleware.SessionMiddleware.__call__",
+            "django.contrib.auth.middleware.AuthenticationMiddleware.__call__",
+            "tests.integrations.django.myapp.settings.TestMiddleware.__call__",
+            "tests.integrations.django.myapp.settings.TestFunctionMiddleware.__call__",
         ]
     else:
         reference_value = [