Defer using the database when calling set_context #716

The collector calls set_context() before any code is run.  If we touch
the database there, it will get created *very* early.  This causes
problems with pytest-cov, which will delete those early-created files
when erasing data.

By deferring the database access until add_lines is called, the data
file stays off the disk until the collection is done (or until the
context switches), which avoids the problem.

3 files changed, 19 insertions, 14 deletions

diff --git a/CHANGES.rst b/CHANGES.rst
index 2ca34b11..e5f4aeb3 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -17,7 +17,9 @@ Change history for Coverage.py
 Unreleased
 ----------
 
-Nothing yet.
+- Bug fixes to context support: `issue 716`_.
+
+.. _issue 716: https://github.com/nedbat/coveragepy/issues/716
 
 
 .. _changes_50a3:
diff --git a/coverage/sqldata.py b/coverage/sqldata.py
index 6c12d681..561878fa 100644
--- a/coverage/sqldata.py
+++ b/coverage/sqldata.py
@@ -93,6 +93,7 @@ class CoverageSqliteData(SimpleReprMixin):
         self._has_lines = False
         self._has_arcs = False
 
+        self._current_context = None
         self._current_context_id = None
 
     def _choose_filename(self):
@@ -205,13 +206,17 @@ class CoverageSqliteData(SimpleReprMixin):
         """Set the current context for future `add_lines` etc."""
         if self._debug.should('dataop'):
             self._debug.write("Setting context: %r" % (context,))
-        self._start_using()
-        context = context or ""
-        with self._connect() as con:
-            row = con.execute("select id from context where context = ?", (context,)).fetchone()
-            if row is not None:
-                self._current_context_id = row[0]
-            else:
+        self._current_context = context
+        self._current_context_id = None
+
+    def _set_context_id(self):
+        """Use the _current_context to set _current_context_id."""
+        context = self._current_context or ""
+        context_id = self._context_id(context)
+        if context_id is not None:
+            self._current_context_id = context_id
+        else:
+            with self._connect() as con:
                 cur = con.execute("insert into context (context) values (?)", (context,))
                 self._current_context_id = cur.lastrowid
 
@@ -229,8 +234,7 @@ class CoverageSqliteData(SimpleReprMixin):
             ))
         self._start_using()
         self._choose_lines_or_arcs(lines=True)
-        if self._current_context_id is None:
-            self.set_context("")
+        self._set_context_id()
         with self._connect() as con:
             for filename, linenos in iitems(line_data):
                 file_id = self._file_id(filename, add=True)
@@ -254,8 +258,7 @@ class CoverageSqliteData(SimpleReprMixin):
             ))
         self._start_using()
         self._choose_lines_or_arcs(arcs=True)
-        if self._current_context_id is None:
-            self.set_context("")
+        self._set_context_id()
         with self._connect() as con:
             for filename, arcs in iitems(arc_data):
                 file_id = self._file_id(filename, add=True)
diff --git a/tests/test_debug.py b/tests/test_debug.py
index 63edc84f..284d9567 100644
--- a/tests/test_debug.py
+++ b/tests/test_debug.py
@@ -144,8 +144,8 @@ class DebugTraceTest(CoverageTest):
             self.assertRegex(real_messages[-1], r"^\s*\d+\.\w{4}: Writing data")
             self.assertRegex(last_line, r"\s+_write_file : .*coverage[/\\]data.py @\d+$")
         else:
-            self.assertRegex(real_messages[-1], r"^\s*\d+\.\w{4}: Adding lines")
-            self.assertRegex(last_line, r"\s+add_lines : .*coverage[/\\]sqldata.py @\d+$")
+            self.assertRegex(real_messages[-1], r"^\s*\d+\.\w{4}: Creating data file")
+            self.assertRegex(last_line, r"\s+_create_db : .*coverage[/\\]sqldata.py @\d+$")
 
     def test_debug_config(self):
         out_lines = self.f1_debug_output(["config"])