feat: Implement "remote only" mode

Closes #1010.

8 files changed, 111 insertions, 23 deletions

diff --git a/doc/MANUAL.adoc b/doc/MANUAL.adoc
index 0e59898f..8577ac1f 100644
--- a/doc/MANUAL.adoc
+++ b/doc/MANUAL.adoc
@@ -881,12 +881,20 @@ temporary files otherwise. You may also want to set <<config_stats,*stats*>> to
     If true, ccache will not use any previously stored result. New results will
     still be cached, possibly overwriting any pre-existing results.
 
+[#config_remote_only]
+*remote_only* (*CCACHE_REMOTE_ONLY* or *CCACHE_NOREMOTE_ONLY*, see _<<Boolean values>>_ above)::
+
+    If true, ccache will only use <<config_remote_storage,remote storage>>. The
+    default is false. Note that cache statistics counters will still be kept in
+    the local cache directory unless <<config_stats,*stats*>> is false.
+
 [#config_remote_storage]
 *remote_storage* (*CCACHE_REMOTE_STORAGE*)::
 
     This option specifies one or several storage backends (separated by space)
-    to query after checking the local cache. See _<<Remote storage backends>>_
-    for documentation of syntax and available backends.
+    to query after checking the local cache (unless
+    <<config_remote_only,*remote_only*>> is true). See _<<Remote storage
+    backends>>_ for documentation of syntax and available backends.
 +
 Examples:
 +
@@ -1034,10 +1042,12 @@ filesystem as the `CCACHE_DIR` path, but this requirement has been relaxed.
 == Remote storage backends
 
 The <<config_remote_storage,*remote_storage*>> option lets you configure ccache
-to use one or several other storage backends in addition to the local cache
-storage located in <<config_cache_dir,*cache_dir*>>. Note that cache statistics
-counters will still be kept in the local cache directory -- remote storage
-backends only store compilation results and manifests.
+to use one or several remote storage backends. By default, the local cache
+directory located in <<config_cache_dir,*cache_dir*>> will be queried first and
+remote storage second, but <<config_remote_only,*remote_only*>> can be set to
+true to disable local storage. Note that cache statistics counters will still be
+kept in the local cache directory -- remote storage backends only store
+compilation results and manifests.
 
 A remote storage backend is specified with a URL, optionally followed by a pipe
 (`|`) and a pipe-separated list of attributes. An attribute is _key_=_value_ or
@@ -1078,7 +1088,8 @@ Examples:
 === Storage interaction
 
 The table below describes the interaction between local and remote storage on
-cache hits and misses:
+cache hits and misses if <<config_remote_only,*remote_only*>> is false (which is
+the default):
 
 [options="header",cols="20%,20%,60%"]
 |==============================================================================
@@ -1094,6 +1105,16 @@ cache hits and misses:
 ^[2]^ Unless local storage is set to share its cache hits with the
 <<config_reshare,*reshare*>> option.
 
+If <<config_remote_only,*remote_only*>> is true:
+
+[options="header",cols="20%,20%,60%"]
+|==============================================================================
+| *Local storage* | *Remote storage* | *What happens*
+
+| - | miss | Compile, write to remote, don't write to local
+| - | hit  | Read from remote, don't write to local
+
+|==============================================================================
 
 === File storage backend
 
diff --git a/src/Config.cpp b/src/Config.cpp
index de5b5247..9d2b3ae4 100644
--- a/src/Config.cpp
+++ b/src/Config.cpp
@@ -91,6 +91,7 @@ enum class ConfigItem {
   read_only,
   read_only_direct,
   recache,
+  remote_only,
   remote_storage,
   reshare,
   run_second_cpp,
@@ -145,6 +146,7 @@ const std::unordered_map<std::string, ConfigKeyTableEntry> k_config_key_table =
     {"read_only", {ConfigItem::read_only}},
     {"read_only_direct", {ConfigItem::read_only_direct}},
     {"recache", {ConfigItem::recache}},
+    {"remote_only", {ConfigItem::remote_only}},
     {"remote_storage", {ConfigItem::remote_storage}},
     {"reshare", {ConfigItem::reshare}},
     {"run_second_cpp", {ConfigItem::run_second_cpp}},
@@ -193,6 +195,7 @@ const std::unordered_map<std::string, std::string> k_env_variable_table = {
   {"READONLY", "read_only"},
   {"READONLY_DIRECT", "read_only_direct"},
   {"RECACHE", "recache"},
+  {"REMOTE_ONLY", "remote_only"},
   {"REMOTE_STORAGE", "remote_storage"},
   {"RESHARE", "reshare"},
   {"SECONDARY_STORAGE", "remote_storage"}, // Alias for CCACHE_REMOTE_STORAGE
@@ -775,6 +778,9 @@ Config::get_string_value(const std::string& key) const
   case ConfigItem::recache:
     return format_bool(m_recache);
 
+  case ConfigItem::remote_only:
+    return format_bool(m_remote_only);
+
   case ConfigItem::remote_storage:
     return m_remote_storage;
 
@@ -1028,6 +1034,10 @@ Config::set_item(const std::string& key,
     m_recache = parse_bool(value, env_var_key, negate);
     break;
 
+  case ConfigItem::remote_only:
+    m_remote_only = parse_bool(value, env_var_key, negate);
+    break;
+
   case ConfigItem::remote_storage:
     m_remote_storage = Util::expand_environment_variables(value);
     break;
diff --git a/src/Config.hpp b/src/Config.hpp
index 7ee59921..7a01b35e 100644
--- a/src/Config.hpp
+++ b/src/Config.hpp
@@ -84,6 +84,7 @@ public:
   bool read_only() const;
   bool read_only_direct() const;
   bool recache() const;
+  bool remote_only() const;
   const std::string& remote_storage() const;
   bool reshare() const;
   bool run_second_cpp() const;
@@ -193,6 +194,7 @@ private:
   bool m_recache = false;
   bool m_reshare = false;
   bool m_run_second_cpp = true;
+  bool m_remote_only = false;
   std::string m_remote_storage;
   core::Sloppiness m_sloppiness;
   bool m_stats = true;
@@ -437,6 +439,12 @@ Config::run_second_cpp() const
   return m_run_second_cpp;
 }
 
+inline bool
+Config::remote_only() const
+{
+  return m_remote_only;
+}
+
 inline const std::string&
 Config::remote_storage() const
 {
diff --git a/src/ccache.cpp b/src/ccache.cpp
index 6cf1c53a..26b4d9fd 100644
--- a/src/ccache.cpp
+++ b/src/ccache.cpp
@@ -901,10 +901,14 @@ write_result(Context& ctx,
 
   core::CacheEntry::Header header(ctx.config, core::CacheEntryType::result);
   const auto cache_entry_data = core::CacheEntry::serialize(header, serializer);
-  const auto raw_files = serializer.get_raw_files();
-  if (!raw_files.empty()) {
-    ctx.storage.local.put_raw_files(result_key, raw_files);
+
+  if (!ctx.config.remote_only()) {
+    const auto& raw_files = serializer.get_raw_files();
+    if (!raw_files.empty()) {
+      ctx.storage.local.put_raw_files(result_key, raw_files);
+    }
   }
+
   ctx.storage.put(result_key, core::CacheEntryType::result, cache_entry_data);
 
   return true;
@@ -1742,7 +1746,7 @@ hash_direct_mode_specific_data(Context& ctx,
       }
     });
   MTR_END("manifest", "manifest_get");
-  if (read_manifests > 1) {
+  if (read_manifests > 1 && !ctx.config.remote_only()) {
     MTR_SCOPE("manifest", "merge");
     LOG("Storing merged manifest {} locally", manifest_key->to_string());
     core::CacheEntry::Header header(ctx.config, core::CacheEntryType::manifest);
diff --git a/src/storage/Storage.cpp b/src/storage/Storage.cpp
index 11490edc..a607be50 100644
--- a/src/storage/Storage.cpp
+++ b/src/storage/Storage.cpp
@@ -237,20 +237,24 @@ Storage::get(const Digest& key,
 {
   MTR_SCOPE("storage", "get");
 
-  auto value = local.get(key, type);
-  local.increment_statistic(value ? core::Statistic::local_storage_hit
-                                  : core::Statistic::local_storage_miss);
-  if (value) {
-    if (m_config.reshare()) {
-      put_in_remote_storage(key, *value, true);
-    }
-    if (entry_receiver(std::move(*value))) {
-      return;
+  if (!m_config.remote_only()) {
+    auto value = local.get(key, type);
+    local.increment_statistic(value ? core::Statistic::local_storage_hit
+                                    : core::Statistic::local_storage_miss);
+    if (value) {
+      if (m_config.reshare()) {
+        put_in_remote_storage(key, *value, true);
+      }
+      if (entry_receiver(std::move(*value))) {
+        return;
+      }
     }
   }
 
   get_from_remote_storage(key, [&](util::Bytes&& data) {
-    local.put(key, type, data, true);
+    if (!m_config.remote_only()) {
+      local.put(key, type, data, true);
+    }
     return entry_receiver(std::move(data));
   });
 }
@@ -262,7 +266,9 @@ Storage::put(const Digest& key,
 {
   MTR_SCOPE("storage", "put");
 
-  local.put(key, type, value);
+  if (!m_config.remote_only()) {
+    local.put(key, type, value);
+  }
   put_in_remote_storage(key, value, false);
 }
 
@@ -271,7 +277,9 @@ Storage::remove(const Digest& key, const core::CacheEntryType type)
 {
   MTR_SCOPE("storage", "remove");
 
-  local.remove(key, type);
+  if (!m_config.remote_only()) {
+    local.remove(key, type);
+  }
   remove_from_remote_storage(key);
 }
 
diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
index 4cd38a38..939a220c 100644
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -54,6 +54,7 @@ addtest(readonly)
 addtest(readonly_direct)
 addtest(remote_file)
 addtest(remote_http)
+addtest(remote_only)
 addtest(remote_redis)
 addtest(remote_redis_unix)
 addtest(remote_url)
diff --git a/test/suites/remote_only.bash b/test/suites/remote_only.bash
new file mode 100644
index 00000000..3dea32ac
--- /dev/null
+++ b/test/suites/remote_only.bash
@@ -0,0 +1,33 @@
+SUITE_remote_only_SETUP() {
+    unset CCACHE_NODIRECT
+    export CCACHE_REMOTE_STORAGE="file:$PWD/remote"
+    export CCACHE_REMOTE_ONLY=1
+
+    generate_code 1 test.c
+}
+
+SUITE_remote_only() {
+    # -------------------------------------------------------------------------
+    TEST "Base case"
+
+    $CCACHE_COMPILE -c test.c
+    expect_stat direct_cache_hit 0
+    expect_stat cache_miss 1
+    expect_stat files_in_cache 0
+    expect_stat local_storage_hit 0
+    expect_stat local_storage_miss 0
+    expect_stat remote_storage_hit 0
+    expect_stat remote_storage_miss 2 # result + manifest
+    expect_file_count 3 '*' remote # CACHEDIR.TAG + result + manifest
+
+    $CCACHE_COMPILE -c test.c
+    expect_stat direct_cache_hit 1
+    expect_stat cache_miss 1
+    expect_stat files_in_cache 0
+    expect_stat local_storage_hit 0
+    expect_stat local_storage_miss 0
+    expect_stat remote_storage_hit 2
+    expect_stat remote_storage_miss 2
+    expect_stat files_in_cache 0
+    expect_file_count 3 '*' remote # CACHEDIR.TAG + result + manifest
+}
diff --git a/unittest/test_Config.cpp b/unittest/test_Config.cpp
index 14456811..0226101f 100644
--- a/unittest/test_Config.cpp
+++ b/unittest/test_Config.cpp
@@ -71,6 +71,7 @@ TEST_CASE("Config: default values")
   CHECK_FALSE(config.read_only());
   CHECK_FALSE(config.read_only_direct());
   CHECK_FALSE(config.recache());
+  CHECK_FALSE(config.remote_only());
   CHECK(config.remote_storage().empty());
   CHECK_FALSE(config.reshare());
   CHECK(config.run_second_cpp());
@@ -413,6 +414,7 @@ TEST_CASE("Config::visit_items")
     "read_only = true\n"
     "read_only_direct = true\n"
     "recache = true\n"
+    "remote_only = true\n"
     "remote_storage = rs\n"
     "reshare = true\n"
     "run_second_cpp = false\n"
@@ -473,6 +475,7 @@ TEST_CASE("Config::visit_items")
     "(test.conf) read_only = true",
     "(test.conf) read_only_direct = true",
     "(test.conf) recache = true",
+    "(test.conf) remote_only = true",
     "(test.conf) remote_storage = rs",
     "(test.conf) reshare = true",
     "(test.conf) run_second_cpp = false",