Don't block within futures in MerkleTreeComputer

Only the outer `buildForFiles`/`buildForSpawn` entry points may block on futures now. This fixes deadlocks that could happen when all threads in `MERKLE_TREE_BUILD_POOL` block on futures scheduled on this pool.

The existing golden test now builds the same Merkle tree multiple times concurrently, which reproduces the deadlock before this change on virtually every run and passes in 100 attempts with the fix.

Fixes bazelbuild#27331

Closes bazelbuild#27345.

PiperOrigin-RevId: 823843469
Change-Id: I3a6fa7e849df4506eed8646e8c93e86515999aae

Author: fmeum

src/main/java/com/google/devtools/build/lib/remote/merkletree/BUILD

@@ -28,6 +28,7 @@ java_library(
         "//src/main/java/com/google/devtools/build/lib/remote/util",
         "//src/main/java/com/google/devtools/build/lib/remote/util:digest_utils",
         "//src/main/java/com/google/devtools/build/lib/skyframe:tree_artifact_value",
+        "//src/main/java/com/google/devtools/build/lib/util:TestType",
         "//src/main/java/com/google/devtools/build/lib/util:string_encoding",
         "//src/main/java/com/google/devtools/build/lib/vfs",
         "//src/main/java/com/google/devtools/build/lib/vfs:pathfragment",