Add special versioning rules for 'release-test/rt/' branches to have shorter versioning. (#19396)

Long versions are sometimes problematic on Windows, because of MAX_PATH
issues. Versions for release builds don't contain the pre-release part, and
are thus usually short, but pre-release versions (which include an arbitrarily
long branch name) can get too long for Windows. This is a complication when
testing a release pipeline/process: we have to use final versioning just for
testing. This isn't ideal, so we special-case branch names that start with
`release-test/rt/`:
  * Example: `iOS 15.1.123-rt` (and nothing else). This makes these versions
    exactly 3 characters longer than the release version, which is hopefully
    enough to avoid MAX_PATH issues on Windows.

Make.config

@@ -153,6 +153,9 @@ NUGET_PRERELEASE_IDENTIFIER=
 else ifeq ($(NUGET_HARDCODED_PRERELEASE_BRANCH_ALPHANUMERIC),$(CURRENT_BRANCH_ALPHANUMERIC))
 # this is a prerelease branch!
 NUGET_PRERELEASE_IDENTIFIER=-$(NUGET_HARDCODED_PRERELEASE_IDENTIFIER)
+else ifeq (release-test/rt/,$(patsubst release-test/rt/%,release-test/rt/,$(CURRENT_BRANCH)))
+# this is a release-testing branch!
+NUGET_PRERELEASE_IDENTIFIER=-rt
 else
 # this is a CI branch!
 NUGET_PRERELEASE_IDENTIFIER=-ci.$(CURRENT_BRANCH_ALPHANUMERIC)

dotnet/VERSIONS.md

@@ -26,6 +26,16 @@ This is the scheme: `OsMajor.OsMinor.InternalRelease[-prereleaseX]+sha.1b2c3d4`.
           referencing `iOS *-xcode13-1.beta.*`
         * It's still possible to sign up for all `xcode13-1` builds, by
           referencing `iOS *-ci.xcode11-3.*`
+    * Long versions are sometimes problematic on Windows, because of MAX_PATH
+      issues. Versions for release builds don't contain the pre-release part,
+      and are thus usually short, but pre-release versions (which include an
+      arbitrarily long branch name) can get too long for Windows. This is a
+      complication when testing a release pipeline/process: we have to use
+      final versioning just for testing. This isn't ideal, so we special-case
+      branch names that start with `release-test/rt/`:
+        * Example: `iOS 15.1.123-rt` (and nothing else). This makes these
+          versions exactly 3 characters longer than the release version, which
+          is hopefully enough to avoid MAX_PATH issues on Windows.
 * Build metadata: Required Hash
     * This is `sha.` + the short commit hash.
         * Use the short hash because the long hash is quite long and