STYLE.md: Discourage the use of seed repos for test datapks-doc-discourage-seeded-test-repos

With the upcoming transition to start supporting the SHA256 object hash
in our repositories we will have to amend our tests to support both SHA1
and SHA256 object hashes. Due to our use of static seed repositories in
many of our tests though this proving to be quite a burden as we have an
abundance of hardcoded SHA1 hashes.

Discourage the use of seed repositories in favor of generating the data
at runtime instead with our test helpers. This gives us agility when the
object format of a repository changes as we don't depend on hardcoded
information anymore, but in many cases it also speeds up test execution
as we don't have to clone thousands of repositories on every test run
anymore. Last but not least this also helps readers understand tests
better given that they don't have to peek into the seed repositories to
get an understanding of how the test data is arranged, but instead can
learn about it by just reading through the now-explicit test data setup.

1 files changed, 15 insertions, 0 deletions

diff --git a/STYLE.md b/STYLE.md
index fbf61a061..667973ee5 100644
--- a/STYLE.md
+++ b/STYLE.md
@@ -243,6 +243,21 @@ tests are supposed to set up required state as part of the tests themselves. All
 `TestMain()` functions must call `testhelper.Run()` though, which performs the
 setup of global state required for tests.
 
+### Test data
+
+Tests should not rely on static Git data but instead generate test data at
+runtime if possible. This is done so that we can easily adapt to changes in
+Git's repository or object format, e.g. in the transition from the SHA1 to the
+SHA256 object hash. Using seed repositories is thus discouraged.
+
+As an alternative, tests can rely on the following helper functions to generate
+their test data:
+
+- `gittest.WriteCommit()`
+- `gittest.WriteTree()`
+- `gittest.WriteBlob()`
+- `gittest.WriteTag()`
+
 ## Black box and white box testing
 
 The dominant style of testing in Gitaly is "white box" testing, meaning