libstore/machines: add enable flag for toml machines

Provide a way to statically disable a machine.
This is fully static and not dynamic.

Change-Id: I394433ad533bca5dbf6a2f7fea22b8aa0f5aad3f

doc/manual/src/advanced-topics/distributed-builds.md

@@ -172,6 +172,10 @@ speed-factor = 2.0
 supported-features = ["kvm", "benchmark"]
 ssh-key = "/home/deepslate/.ssh/id_ed25519"
 
+[machines.legacy]
+uri = "ssh://nix@nix-15-11.nixos.org"
+enable = false
+
 ```
 
 > **Note**
@@ -183,6 +187,13 @@ For testing purposes, one can also define a builder ad hoc on the CLI as follows
 `--builders 'machines.andesite = {uri = "ssh://lix@andesite.lix.systems", jobs = 8}'`
 
 
+#### Special handling of fields
+ - `enable` (**optional**)
+    If set to false, the declared machine will not be loaded.
+    This allows one to statically disable machines.
+
+    Defaults to true
+
 ### Using the legacy format
 > **Warning**
 >

lix/libstore/machines.cc

@@ -242,6 +242,7 @@ static const std::set<std::string> EXPECTED_KEYS = {
     "supported-features",
     "mandatory-features",
     "ssh-public-host-key",
+    "enable",
 };
 
 static toml::result<float, std::string> getSpeedFactor(const toml::value & data)
@@ -408,8 +409,17 @@ static toml::result<Machines, std::vector<std::string>> parseToml(const toml::va
             auto err = res.as_err();
             parserErrors.push_back(fmt("for machine %s:", name));
             parserErrors.insert(parserErrors.end(), err.begin(), err.end());
+            continue;
+        }
+        auto enable = parse<bool>(machine, "enable", true);
+        if (enable.is_ok()) {
+            if (enable.unwrap()) {
+                // Check if it hasn't been statically disabled
+                // But still throw parsing errors if it was
+                machines.push_back(res.unwrap());
+            }
         } else {
-            machines.push_back(res.unwrap());
+            parserErrors.push_back(enable.as_err());
         }
     }
 

tests/unit/libstore/machines_toml.cc

@@ -233,6 +233,21 @@ TEST(machines, getMachinesTOMLWithIncorrectTyping)
 
     settings.builders.override("machines.a = \"lix@andesite.lix.sytems\"\n");
     EXPECT_MESSAGE_THROW(getMachines(), UsageError, "Each machine must be a table");
+
+    settings.builders.override(
+        "version = \"1\"\n"
+        "[machines.scratchy]\n"
+        "uri = \"nix@scratchy.labs.cs.uu.nl\"\n"
+    );
+    EXPECT_MESSAGE_THROW(getMachines(), UsageError, "bad_cast to integer");
+
+    settings.builders.override(
+        "version = 1\n"
+        "[machines.legacy]\n"
+        "uri = \"ssh://nix@nix-15-11.nixos.org\"\n"
+        "enable = 0\n"
+    );
+    EXPECT_MESSAGE_THROW(getMachines(), UsageError, "bad_cast to boolean");
 }
 
 TEST(machines, getMachinesTOMLBadVersion)
@@ -282,4 +297,20 @@ TEST(machines, getMachinesTOMLInvalidSyntaxButClearlyTOML)
     EXPECT_MESSAGE_THROW(getMachines(), UsageError, "invalid Machines TOML syntax:");
 }
 
+TEST(machines, getMachinesTOMLOneDisabled)
+{
+    settings.builders.override(
+        "version = 1\n"
+        "[machines.a]\n"
+        "uri = \"ssh://test\"\n"
+        "enable = false\n"
+        "\n"
+        "[machines.b]\n"
+        "uri = \"ssh://test2\"\n"
+    );
+    auto actual = getMachines();
+    ASSERT_THAT(actual, SizeIs(1));
+    EXPECT_THAT(actual[0], Field(&Machine::storeUri, EndsWith("test2")));
+}
+
 #undef EXPECT_MESSAGE_THROW