document code coverage options (near#10737)

docs/SUMMARY.md

@@ -50,6 +50,7 @@
 - [Testing](./practices/testing/README.md)
   - [Python Tests](./practices/testing/python_tests.md)
   - [Testing Utils](./practices/testing/test_utils.md)
+  - [Test Coverage](./practices/testing/coverage.md)
 - [Protocol Upgrade](./practices/protocol_upgrade.md)
 
 # Advanced configuration

docs/practices/testing/coverage.md

@@ -0,0 +1,55 @@
+# Test Coverage
+
+In order to focus the testing effort where it is most needed, we have a few
+ways we track test coverage.
+
+## Codecov
+
+The main one is Codecov. Coverage is visible [on this
+webpage](https://app.codecov.io/gh/near/nearcore), and displays the total
+coverage, including unit and integration tests. Codecov is especially
+interesting for [its PR
+comments](https://github.com/near/nearcore/pull/10731#issuecomment-1985356880).
+The PR comments, in particular, can easily show which diff lines are being
+tested and which are not.
+
+However, sometimes Codecov gives too rough estimates, and this is where
+artifact results come in.
+
+## Artifact Results
+
+We also push artifacts, as a result of each CI run. You can access them here:
+1. Click "Details" on one of the CI actions run on your PR (literally any one
+   of the actions is fine, you can also access CI actions runs on any CI)
+2. Click "Summary" on the top left of the opening page
+3. Scroll to the bottom of the page
+4. In the "Artifacts" section, just above the "Summary" section, there is a
+   `coverage-html` link (there is also `coverage-lcov` for people who use eg.
+   the coverage gutters vscode integration)
+5. Downloading it will give you a zip file with the interesting files.
+
+In there, you can find:
+- Two `-diff` files, that contain code coverage for the diff of your PR, to
+  easily see exactly which lines are covered and which are not
+- Two `-full` folders, that contain code coverage for the whole repository
+- Each of these exists in one `unit-` variant, that only contains the unit
+  tests, and one `integration-` variant, that contains all the tests we
+  currently have
+
+
+**To check that your PR is properly tested**, if you want better quality
+coverage than what codecov "requires," you can have a look at `unit-diff`,
+because we agreed that we want unit tests to be able to detect most bugs
+due to the troubles of debugging failing integration tests.
+
+**To find a place that would deserve adding more tests**, look at one of the
+`-full` directories on master, pick one not-well-tested file, and add (ideally
+unit) tests for the lines that are missing.
+
+The presentation is unfortunately less easy to access than codecov, and less
+eye-catchy. On the other hand, it should be more precise. In particular, the
+`-full` variants show region-based coverage. It can tell you that eg. the `?`
+branch is not covered properly by highlighting it red.
+
+One caveat to be aware of: the `-full` variants do not highlight covered lines
+in green, they just highlight non-covered lines in red.