computed roots: restrict in-memory root computation to the supported cases

... and, in this way, avoid git-tree computations with invalid git
identifiers.

3 files changed, 12 insertions, 2 deletions

diff --git a/src/buildtool/computed_roots/TARGETS b/src/buildtool/computed_roots/TARGETS
index 208711df..a714cfc1 100644
--- a/src/buildtool/computed_roots/TARGETS
+++ b/src/buildtool/computed_roots/TARGETS
@@ -70,6 +70,7 @@
     , ["src/buildtool/build_engine/target_map", "configured_target"]
     , ["src/buildtool/common", "cli"]
     , ["src/buildtool/common", "common"]
+    , ["src/buildtool/crypto", "hash_function"]
     , ["src/buildtool/file_system", "file_root"]
     , ["src/buildtool/file_system", "file_system_manager"]
     , ["src/buildtool/file_system", "git_cas"]
diff --git a/src/buildtool/computed_roots/artifacts_root.cpp b/src/buildtool/computed_roots/artifacts_root.cpp
index dde240a6..8124461b 100644
--- a/src/buildtool/computed_roots/artifacts_root.cpp
+++ b/src/buildtool/computed_roots/artifacts_root.cpp
@@ -146,8 +146,6 @@ auto ArtifactsRoot(ExpressionPtr const& stage,
                   true);
         return std::nullopt;
     }
-    // TODO(aehlig): Do re-hashing in case of compatible mode
-
     // We obtain the entries odered by key; in particular, the entries of all
     // subtrees will be next to each other. So, we compute the final tree
     // keeping a stack of partially set up tree while walking.
diff --git a/src/buildtool/computed_roots/lookup_cache.cpp b/src/buildtool/computed_roots/lookup_cache.cpp
index 9021463c..000cdbee 100644
--- a/src/buildtool/computed_roots/lookup_cache.cpp
+++ b/src/buildtool/computed_roots/lookup_cache.cpp
@@ -27,6 +27,7 @@
 #include "src/buildtool/build_engine/expression/configuration.hpp"
 #include "src/buildtool/build_engine/expression/target_result.hpp"
 #include "src/buildtool/computed_roots/artifacts_root.hpp"
+#include "src/buildtool/crypto/hash_function.hpp"
 #include "src/buildtool/file_system/file_root.hpp"
 #include "src/buildtool/multithreading/task_system.hpp"
 #include "src/buildtool/storage/target_cache_entry.hpp"
@@ -101,6 +102,16 @@ auto LookupCache(BuildMaps::Target::ConfiguredTarget const& ctarget,
     if (not flexible_vars) {
         return unexpected(std::monostate{});
     }
+    // TODO(aehlig): Support re-hashing in case of compatible mode, so that
+    // cache lookups can be used in this case as well.
+    if (storage.GetHashFunction().GetType() != HashFunction::Type::GitSHA1) {
+        // So far, transforming the result of a cache hit in memory into
+        // a git root is only implemented for GitSHA1. Therefore, do not
+        // try to look up cache hits, and simply pretend a non hit.
+        return expected<std::optional<std::string>, std::monostate>(
+            std::nullopt);
+    }
+
     auto effective_config = ctarget.config.Prune(*flexible_vars);
     auto cache_key = storage.TargetCache().ComputeKey(
         *repo_key, ctarget.target.GetNamedTarget(), effective_config);