Factor out a LogStore interface

Continue progress on #5729.

Just as I hoped, this uncovered an issue: the daemon protocol is missing
a way to query build logs. This doesn't effect `unix://`, but does
effect `ssh://`. A FIXME is left for this, so we come back to it later.

src/libstore/binary-cache-store.hh

@@ -2,6 +2,7 @@
 
 #include "crypto.hh"
 #include "store-api.hh"
+#include "log-store.hh"
 
 #include "pool.hh"
 
@@ -28,7 +29,9 @@ struct BinaryCacheStoreConfig : virtual StoreConfig
         "other than -1 which we reserve to indicate Nix defaults should be used"};
 };
 
-class BinaryCacheStore : public virtual BinaryCacheStoreConfig, public virtual Store
+class BinaryCacheStore : public virtual BinaryCacheStoreConfig,
+    public virtual Store,
+    public virtual LogStore
 {
 
 private:

src/libstore/build/local-derivation-goal.cc

@@ -1340,6 +1340,12 @@ struct RestrictedStore : public virtual RestrictedStoreConfig, public virtual Lo
         next->queryMissing(allowed, willBuild, willSubstitute,
             unknown, downloadSize, narSize);
     }
+
+    virtual std::optional<std::string> getBuildLog(const StorePath & path) override
+    { return std::nullopt; }
+
+    virtual void addBuildLog(const StorePath & path, std::string_view log) override
+    { unsupported("addBuildLog"); }
 };
 
 

src/libstore/daemon.cc

@@ -4,6 +4,7 @@
 #include "build-result.hh"
 #include "store-api.hh"
 #include "gc-store.hh"
+#include "log-store.hh"
 #include "path-with-outputs.hh"
 #include "finally.hh"
 #include "archive.hh"
@@ -953,11 +954,12 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
         logger->startWork();
         if (!trusted)
             throw Error("you are not privileged to add logs");
+        auto & logStore = LogStore::require(*store);
         {
             FramedSource source(from);
             StringSink sink;
             source.drainInto(sink);
-            store->addBuildLog(path, sink.s);
+            logStore.addBuildLog(path, sink.s);
         }
         logger->stopWork();
         to << 1;

src/libstore/local-fs-store.hh

@@ -2,6 +2,7 @@
 
 #include "store-api.hh"
 #include "gc-store.hh"
+#include "log-store.hh"
 
 namespace nix {
 
@@ -24,7 +25,10 @@ struct LocalFSStoreConfig : virtual StoreConfig
         "physical path to the Nix store"};
 };
 
-class LocalFSStore : public virtual LocalFSStoreConfig, public virtual Store, virtual GcStore
+class LocalFSStore : public virtual LocalFSStoreConfig,
+    public virtual Store,
+    public virtual GcStore,
+    public virtual LogStore
 {
 public:
 

src/libstore/log-store.cc

@@ -0,0 +1,13 @@
+#include "log-store.hh"
+
+namespace nix {
+
+LogStore & LogStore::require(Store & store)
+{
+    auto * gcStore = dynamic_cast<LogStore *>(&store);
+    if (!gcStore)
+        throw UsageError("Build log storage and retrieval not supported by store '%s'", store.getUri());
+    return *gcStore;
+}
+
+}

src/libstore/log-store.hh

@@ -0,0 +1,19 @@
+#pragma once
+
+#include "store-api.hh"
+
+
+namespace nix {
+
+struct LogStore : public virtual Store
+{
+    /* Return the build log of the specified store path, if available,
+       or null otherwise. */
+    virtual std::optional<std::string> getBuildLog(const StorePath & path) = 0;
+
+    virtual void addBuildLog(const StorePath & path, std::string_view log) = 0;
+
+    static LogStore & require(Store & store);
+};
+
+}

src/libstore/remote-store.hh

@@ -5,6 +5,7 @@
 
 #include "store-api.hh"
 #include "gc-store.hh"
+#include "log-store.hh"
 
 
 namespace nix {
@@ -30,7 +31,10 @@ struct RemoteStoreConfig : virtual StoreConfig
 
 /* FIXME: RemoteStore is a misnomer - should be something like
    DaemonStore. */
-class RemoteStore : public virtual RemoteStoreConfig, public virtual Store, public virtual GcStore
+class RemoteStore : public virtual RemoteStoreConfig,
+    public virtual Store,
+    public virtual GcStore,
+    public virtual LogStore
 {
 public:
 

src/libstore/ssh-store.cc

@@ -52,6 +52,10 @@ public:
     bool sameMachine() override
     { return false; }
 
+    // FIXME extend daemon protocol, move implementation to RemoteStore
+    std::optional<std::string> getBuildLog(const StorePath & path) override
+    { unsupported("getBuildLog"); }
+
 private:
 
     struct Connection : RemoteStore::Connection

src/libstore/store-api.hh

@@ -605,14 +605,6 @@ public:
      */
     StorePathSet exportReferences(const StorePathSet & storePaths, const StorePathSet & inputPaths);
 
-    /* Return the build log of the specified store path, if available,
-       or null otherwise. */
-    virtual std::optional<std::string> getBuildLog(const StorePath & path)
-    { return std::nullopt; }
-
-    virtual void addBuildLog(const StorePath & path, std::string_view log)
-    { unsupported("addBuildLog"); }
-
     /* Hack to allow long-running processes like hydra-queue-runner to
        occasionally flush their path info cache. */
     void clearPathInfoCache()