Join continuation lines in a builder

[iderex]

Refs #375, the class cs/string-concatenation-in-loop: one alert, in the continuation join of the script reader ReleaseVersionAgreementTests walks a step's shell with.

Finishes: #375

The site

Read from the platform at the head this branch starts from:

git rev-parse origin/master
e941800b27ee2944bc0759cef29a403e4f98e00d

gh api repos/Flowfin/jellyfin-plugin-metadata-sync/code-scanning/alerts/237 --jq '"#\(.number) \(.state) \(.rule.id) \(.most_recent_instance.location.path):\(.most_recent_instance.location.start_line) sha=\(.most_recent_instance.commit_sha[0:7])"'
#237 open cs/string-concatenation-in-loop Jellyfin.Plugin.MetadataSync.Tests/ReleaseVersionAgreementTests.cs:737 sha=e941800

The line is the pending += ... inside Script, which glues the halves of a shell line ending in a backslash back together before the readings above it look for a refusal or an assignment.

What changed

pending is a StringBuilder. A continuation line appends its text and a space, a complete line is appended and the joined text taken out, and the builder is cleared rather than replaced. What the reader returns does not move, and nothing outside this one function is touched.

Proved by breaking it

The suite is what says the join kept its meaning, on both lines:

dotnet build Jellyfin.Plugin.MetadataSync.sln --nologo
VSTEST_CONNECTION_TIMEOUT=600 DOTNET_CLI_UI_LANGUAGE=en dotnet test Jellyfin.Plugin.MetadataSync.Tests/Jellyfin.Plugin.MetadataSync.Tests.csproj --nologo --no-build -f net9.0
Failed: 0, Passed: 1709, Skipped: 0, Total: 1709
... -f net10.0
Failed: 0, Passed: 1709, Skipped: 0, Total: 1709

Then the two ways this rewrite could go wrong, each done to the reader and restored, with the two route classes run on net9.0:

the continuation branch made unreachable, so nothing is joined   -> Failed: 1, Passed: 18: TheAssemblyIsRefusedWhenItIsStampedWithAnotherVersion
the space between the two halves dropped                         -> Failed: 0, Passed: 19
restored                                                         -> Failed: 0, Passed: 19

The first is red on the route itself rather than on a fixture: the assembly step in publish.yaml reads its assignment across a continuation, so a reader that stops joining takes half a command as the assignment's origin and the refusal is read as comparing something else.

The second stays green, and that is disclosed rather than repaired. Every reading downstream of the join tolerates the missing space: the refusal patterns allow zero whitespace at every place a continuation would split them, and an origin is a substring read. So the space is held by nothing, and a fixture that made it bite would have to split a line in a place no shell script splits one, which would be a fixture for the reader and not for the route. What holds the space is the reading.

The means

C#, in the file the alert names. StringBuilder is in the base library and the test project already uses it in five other files. The alternative was a dismissal; the sibling board that met this class took the same repair, and the query read the site correctly.

What this does not say

The alert closes when the analysis next runs over master, not on this merge, so #375 stays open until that reading is pasted into it. Whether this class arrives again is operations#2176's question and not this change's.

No second reader looked at this. The runs above are the evidence in place of one.