nix flake metadata: print modified dates for input flakes

This was always in the lock file and we can simply actually print it.

The test for this is a little bit silly but it should correctly
control for my daring to exercise timezone code *and* locale code in a
test, which I strongly suspect nobody dared do before.

Sample (abridged):
```
Path:          /nix/store/gaxb42z68bcr8lch467shvmnhjjzgd8b-source
Last modified: 1970-01-01 00:16:40
Inputs:
├───flake-compat: github:edolstra/flake-compat/0f9255e01c2351cc7d116c072cb317785dd33b33
│   Last modified: 2023-10-04 13:37:54
├───flake-utils: github:numtide/flake-utils/b1d9ab70662946ef0850d488da1c9019f3a9752a
│   Last modified: 2024-03-11 08:33:50
│   └───systems: github:nix-systems/default/da67096a3b9bf56a91d16901293e51ba5b49a27e
│       Last modified: 2023-04-09 08:27:08
```

Change-Id: I355f82cb4b633974295375ebad646fb6e2107f9b

6 files changed, 366 insertions, 6 deletions

diff --git a/doc/manual/rl-next/flake-metadata-time.md b/doc/manual/rl-next/flake-metadata-time.md
new file mode 100644
index 000000000..8664b5a75
--- /dev/null
+++ b/doc/manual/rl-next/flake-metadata-time.md
@@ -0,0 +1,28 @@
+---
+synopsis: "`nix flake metadata` prints modified date"
+cls: 1700
+credits: jade
+category: Improvements
+---
+
+Ever wonder "gee, when *did* I update nixpkgs"?
+Wonder no more, because `nix flake metadata` now simply tells you the times every locked flake input was updated:
+
+```
+<...>
+Description:   The purely functional package manager
+Path:          /nix/store/c91yi8sxakc2ry7y4ac1smzwka4l5p78-source
+Revision:      c52cff582043838bbe29768e7da232483d52b61d-dirty
+Last modified: 2024-07-31 22:15:54
+Inputs:
+├───flake-compat: github:edolstra/flake-compat/0f9255e01c2351cc7d116c072cb317785dd33b33
+│   Last modified: 2023-10-04 06:37:54
+├───nix2container: github:nlewo/nix2container/3853e5caf9ad24103b13aa6e0e8bcebb47649fe4
+│   Last modified: 2024-07-10 13:15:56
+├───nixpkgs: github:NixOS/nixpkgs/e21630230c77140bc6478a21cd71e8bb73706fce
+│   Last modified: 2024-07-25 11:26:27
+├───nixpkgs-regression: github:NixOS/nixpkgs/215d4d0fd80ca5163643b03a33fde804a29cc1e2
+│   Last modified: 2022-01-24 11:20:45
+└───pre-commit-hooks: github:cachix/git-hooks.nix/f451c19376071a90d8c58ab1a953c6e9840527fd
+    Last modified: 2024-07-15 04:21:09
+```
diff --git a/src/nix/flake.cc b/src/nix/flake.cc
index 9d18b81b8..672930342 100644
--- a/src/nix/flake.cc
+++ b/src/nix/flake.cc
@@ -209,6 +209,11 @@ struct CmdFlakeMetadata : FlakeCommand, MixJSON
     {
         auto lockedFlake = lockFlake();
         auto & flake = lockedFlake.flake;
+        auto formatTime = [](time_t time) -> std::string {
+            std::ostringstream os{};
+            os << std::put_time(std::localtime(&time), "%F %T");
+            return os.str();
+        };
 
         if (json) {
             nlohmann::json j;
@@ -260,7 +265,7 @@ struct CmdFlakeMetadata : FlakeCommand, MixJSON
             if (auto lastModified = flake.lockedRef.input.getLastModified())
                 logger->cout(
                     ANSI_BOLD "Last modified:" ANSI_NORMAL " %s",
-                    std::put_time(std::localtime(&*lastModified), "%F %T"));
+                    formatTime(*lastModified));
 
             if (!lockedFlake.lockFile.root->inputs.empty())
                 logger->cout(ANSI_BOLD "Inputs:" ANSI_NORMAL);
@@ -275,16 +280,25 @@ struct CmdFlakeMetadata : FlakeCommand, MixJSON
                     bool last = i + 1 == node.inputs.size();
 
                     if (auto lockedNode = std::get_if<0>(&input.second)) {
-                        logger->cout("%s" ANSI_BOLD "%s" ANSI_NORMAL ": %s",
-                            prefix + (last ? treeLast : treeConn), input.first,
+                        // ├───agenix: github:ryantm/agenix/8d37c5bdeade12b6479c85acd133063ab53187a0
+                        logger->cout("%s%s" ANSI_BOLD "%s" ANSI_NORMAL ": %s",
+                            prefix, last ? treeLast : treeConn, input.first,
                             (*lockedNode)->lockedRef);
 
+                        // ├───lix: https://git.lix.systems/api/v1/repos/lix-project <....>
+                        // │   Last modified: 2024-07-31 21:01:34
+                        if (auto lastModified = (*lockedNode)->lockedRef.input.getLastModified()) {
+                            logger->cout("%s%s" ANSI_BOLD "%s" ANSI_NORMAL ": %s",
+                                prefix, last ? treeNull : treeLine, "Last modified", formatTime(*lastModified));
+                        }
+
                         bool firstVisit = visited.insert(*lockedNode).second;
 
                         if (firstVisit) recurse(**lockedNode, prefix + (last ? treeNull : treeLine));
                     } else if (auto follows = std::get_if<1>(&input.second)) {
-                        logger->cout("%s" ANSI_BOLD "%s" ANSI_NORMAL " follows input '%s'",
-                            prefix + (last ? treeLast : treeConn), input.first,
+                        // │   ├───darwin follows input 'flake-utils'
+                        logger->cout("%s%s" ANSI_BOLD "%s" ANSI_NORMAL " follows input '%s'",
+                            prefix, last ? treeLast : treeConn, input.first,
                             printInputPath(*follows));
                     }
                 }
diff --git a/tests/functional/flakes/flake-metadata.sh b/tests/functional/flakes/flake-metadata.sh
new file mode 100644
index 000000000..ab5a69b3a
--- /dev/null
+++ b/tests/functional/flakes/flake-metadata.sh
@@ -0,0 +1,36 @@
+source ./common.sh
+
+flakeDir=$TEST_ROOT/flake
+mkdir -p "$flakeDir"
+
+cat > "$flakeDir/flake.nix" <<-'EOF'
+{
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable-small";
+    flake-utils.url = "github:numtide/flake-utils";
+    flake-compat = {
+      url = "github:edolstra/flake-compat";
+      flake = false;
+    };
+    lanzaboote = {
+      url = "github:nix-community/lanzaboote";
+      inputs.nixpkgs.follows = "nixpkgs";
+      inputs.flake-utils.follows = "flake-utils";
+      inputs.flake-compat.follows = "flake-compat";
+    };
+  };
+
+  outputs = { ... }: {};
+}
+EOF
+
+cp flake-metadata/flake.lock "$flakeDir"
+touch -d @1000 "$flakeDir/flake.nix" "$flakeDir/flake.lock" "$flakeDir"
+
+# For some reason we use NIX_STORE_DIR which causes unstable paths. This is
+# goofy. We can just use --store, which sets rootDir and does not have this
+# problem.
+actualStore=$NIX_STORE_DIR
+unset NIX_STORE_DIR
+NOCOLOR=1 TZ=UTC LC_ALL=C.UTF-8 nix flake metadata --store "$actualStore" "$flakeDir" | grep -v -e 'Locked URL:' -e 'Resolved URL:' > "$TEST_ROOT/metadata.out.actual"
+diff -u flake-metadata/metadata.out.expected "$TEST_ROOT/metadata.out.actual"
diff --git a/tests/functional/flakes/flake-metadata/flake.lock b/tests/functional/flakes/flake-metadata/flake.lock
new file mode 100644
index 000000000..62b7dc3e8
--- /dev/null
+++ b/tests/functional/flakes/flake-metadata/flake.lock
@@ -0,0 +1,245 @@
+{
+  "nodes": {
+    "crane": {
+      "inputs": {
+        "nixpkgs": [
+          "lanzaboote",
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1711299236,
+        "narHash": "sha256-6/JsyozOMKN8LUGqWMopKTSiK8N79T8Q+hcxu2KkTXg=",
+        "owner": "ipetkov",
+        "repo": "crane",
+        "rev": "880573f80d09e18a11713f402b9e6172a085449f",
+        "type": "github"
+      },
+      "original": {
+        "owner": "ipetkov",
+        "repo": "crane",
+        "type": "github"
+      }
+    },
+    "flake-compat": {
+      "flake": false,
+      "locked": {
+        "lastModified": 1696426674,
+        "narHash": "sha256-kvjfFW7WAETZlt09AgDn1MrtKzP7t90Vf7vypd3OL1U=",
+        "owner": "edolstra",
+        "repo": "flake-compat",
+        "rev": "0f9255e01c2351cc7d116c072cb317785dd33b33",
+        "type": "github"
+      },
+      "original": {
+        "owner": "edolstra",
+        "repo": "flake-compat",
+        "type": "github"
+      }
+    },
+    "flake-parts": {
+      "inputs": {
+        "nixpkgs-lib": [
+          "lanzaboote",
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1709336216,
+        "narHash": "sha256-Dt/wOWeW6Sqm11Yh+2+t0dfEWxoMxGBvv3JpIocFl9E=",
+        "owner": "hercules-ci",
+        "repo": "flake-parts",
+        "rev": "f7b3c975cf067e56e7cda6cb098ebe3fb4d74ca2",
+        "type": "github"
+      },
+      "original": {
+        "owner": "hercules-ci",
+        "repo": "flake-parts",
+        "type": "github"
+      }
+    },
+    "flake-utils": {
+      "inputs": {
+        "systems": "systems"
+      },
+      "locked": {
+        "lastModified": 1710146030,
+        "narHash": "sha256-SZ5L6eA7HJ/nmkzGG7/ISclqe6oZdOZTNoesiInkXPQ=",
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "rev": "b1d9ab70662946ef0850d488da1c9019f3a9752a",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "type": "github"
+      }
+    },
+    "gitignore": {
+      "inputs": {
+        "nixpkgs": [
+          "lanzaboote",
+          "pre-commit-hooks-nix",
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1709087332,
+        "narHash": "sha256-HG2cCnktfHsKV0s4XW83gU3F57gaTljL9KNSuG6bnQs=",
+        "owner": "hercules-ci",
+        "repo": "gitignore.nix",
+        "rev": "637db329424fd7e46cf4185293b9cc8c88c95394",
+        "type": "github"
+      },
+      "original": {
+        "owner": "hercules-ci",
+        "repo": "gitignore.nix",
+        "type": "github"
+      }
+    },
+    "lanzaboote": {
+      "inputs": {
+        "crane": "crane",
+        "flake-compat": [
+          "flake-compat"
+        ],
+        "flake-parts": "flake-parts",
+        "flake-utils": [
+          "flake-utils"
+        ],
+        "nixpkgs": [
+          "nixpkgs"
+        ],
+        "pre-commit-hooks-nix": "pre-commit-hooks-nix",
+        "rust-overlay": "rust-overlay"
+      },
+      "locked": {
+        "lastModified": 1713369831,
+        "narHash": "sha256-G4OGxvlIIjphpkxcRAkf1QInYsAeqbfNh6Yl1JLy2uM=",
+        "owner": "nix-community",
+        "repo": "lanzaboote",
+        "rev": "850f27322239f8cfa56b122cc9a278ab99a49015",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-community",
+        "repo": "lanzaboote",
+        "type": "github"
+      }
+    },
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1719824438,
+        "narHash": "sha256-pY0wosAgcr9W4vmGML0T3BVhQiGuKoozCbs2t+Je1zc=",
+        "owner": "nixos",
+        "repo": "nixpkgs",
+        "rev": "7f993cdf26ccef564eabf31fdb40d140821e12bc",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nixos",
+        "ref": "nixos-unstable-small",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "nixpkgs-stable": {
+      "locked": {
+        "lastModified": 1710695816,
+        "narHash": "sha256-3Eh7fhEID17pv9ZxrPwCLfqXnYP006RKzSs0JptsN84=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "614b4613980a522ba49f0d194531beddbb7220d3",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-23.11",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "pre-commit-hooks-nix": {
+      "inputs": {
+        "flake-compat": [
+          "lanzaboote",
+          "flake-compat"
+        ],
+        "flake-utils": [
+          "lanzaboote",
+          "flake-utils"
+        ],
+        "gitignore": "gitignore",
+        "nixpkgs": [
+          "lanzaboote",
+          "nixpkgs"
+        ],
+        "nixpkgs-stable": "nixpkgs-stable"
+      },
+      "locked": {
+        "lastModified": 1710923068,
+        "narHash": "sha256-6hOpUiuxuwpXXc/xfJsBUJeqqgGI+JMJuLo45aG3cKc=",
+        "owner": "cachix",
+        "repo": "pre-commit-hooks.nix",
+        "rev": "e611897ddfdde3ed3eaac4758635d7177ff78673",
+        "type": "github"
+      },
+      "original": {
+        "owner": "cachix",
+        "repo": "pre-commit-hooks.nix",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "flake-compat": "flake-compat",
+        "flake-utils": "flake-utils",
+        "lanzaboote": "lanzaboote",
+        "nixpkgs": "nixpkgs"
+      }
+    },
+    "rust-overlay": {
+      "inputs": {
+        "flake-utils": [
+          "lanzaboote",
+          "flake-utils"
+        ],
+        "nixpkgs": [
+          "lanzaboote",
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1711246447,
+        "narHash": "sha256-g9TOluObcOEKewFo2fR4cn51Y/jSKhRRo4QZckHLop0=",
+        "owner": "oxalica",
+        "repo": "rust-overlay",
+        "rev": "dcc802a6ec4e9cc6a1c8c393327f0c42666f22e4",
+        "type": "github"
+      },
+      "original": {
+        "owner": "oxalica",
+        "repo": "rust-overlay",
+        "type": "github"
+      }
+    },
+    "systems": {
+      "locked": {
+        "lastModified": 1681028828,
+        "narHash": "sha256-Vy1rq5AaRuLzOxct8nz4T6wlgyUR7zLU309k9mBC768=",
+        "owner": "nix-systems",
+        "repo": "default",
+        "rev": "da67096a3b9bf56a91d16901293e51ba5b49a27e",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-systems",
+        "repo": "default",
+        "type": "github"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}
diff --git a/tests/functional/flakes/flake-metadata/metadata.out.expected b/tests/functional/flakes/flake-metadata/metadata.out.expected
new file mode 100644
index 000000000..72e57eca7
--- /dev/null
+++ b/tests/functional/flakes/flake-metadata/metadata.out.expected
@@ -0,0 +1,36 @@
+Path:          /nix/store/gaxb42z68bcr8lch467shvmnhjjzgd8b-source
+Last modified: 1970-01-01 00:16:40
+Inputs:
+├───flake-compat: github:edolstra/flake-compat/0f9255e01c2351cc7d116c072cb317785dd33b33
+│   Last modified: 2023-10-04 13:37:54
+├───flake-utils: github:numtide/flake-utils/b1d9ab70662946ef0850d488da1c9019f3a9752a
+│   Last modified: 2024-03-11 08:33:50
+│   └───systems: github:nix-systems/default/da67096a3b9bf56a91d16901293e51ba5b49a27e
+│       Last modified: 2023-04-09 08:27:08
+├───lanzaboote: github:nix-community/lanzaboote/850f27322239f8cfa56b122cc9a278ab99a49015
+│   Last modified: 2024-04-17 16:03:51
+│   ├───crane: github:ipetkov/crane/880573f80d09e18a11713f402b9e6172a085449f
+│   │   Last modified: 2024-03-24 16:53:56
+│   │   └───nixpkgs follows input 'lanzaboote/nixpkgs'
+│   ├───flake-compat follows input 'flake-compat'
+│   ├───flake-parts: github:hercules-ci/flake-parts/f7b3c975cf067e56e7cda6cb098ebe3fb4d74ca2
+│   │   Last modified: 2024-03-01 23:36:56
+│   │   └───nixpkgs-lib follows input 'lanzaboote/nixpkgs'
+│   ├───flake-utils follows input 'flake-utils'
+│   ├───nixpkgs follows input 'nixpkgs'
+│   ├───pre-commit-hooks-nix: github:cachix/pre-commit-hooks.nix/e611897ddfdde3ed3eaac4758635d7177ff78673
+│   │   Last modified: 2024-03-20 08:24:28
+│   │   ├───flake-compat follows input 'lanzaboote/flake-compat'
+│   │   ├───flake-utils follows input 'lanzaboote/flake-utils'
+│   │   ├───gitignore: github:hercules-ci/gitignore.nix/637db329424fd7e46cf4185293b9cc8c88c95394
+│   │   │   Last modified: 2024-02-28 02:28:52
+│   │   │   └───nixpkgs follows input 'lanzaboote/pre-commit-hooks-nix/nixpkgs'
+│   │   ├───nixpkgs follows input 'lanzaboote/nixpkgs'
+│   │   └───nixpkgs-stable: github:NixOS/nixpkgs/614b4613980a522ba49f0d194531beddbb7220d3
+│   │       Last modified: 2024-03-17 17:16:56
+│   └───rust-overlay: github:oxalica/rust-overlay/dcc802a6ec4e9cc6a1c8c393327f0c42666f22e4
+│       Last modified: 2024-03-24 02:14:07
+│       ├───flake-utils follows input 'lanzaboote/flake-utils'
+│       └───nixpkgs follows input 'lanzaboote/nixpkgs'
+└───nixpkgs: github:nixos/nixpkgs/7f993cdf26ccef564eabf31fdb40d140821e12bc
+    Last modified: 2024-07-01 09:00:38
diff --git a/tests/functional/meson.build b/tests/functional/meson.build
index 7a9c7182f..2b5dfe422 100644
--- a/tests/functional/meson.build
+++ b/tests/functional/meson.build
@@ -69,8 +69,9 @@ functional_tests_scripts = [
   'flakes/unlocked-override.sh',
   'flakes/absolute-paths.sh',
   'flakes/build-paths.sh',
-  'flakes/flake-registry.sh',
   'flakes/flake-in-submodule.sh',
+  'flakes/flake-metadata.sh',
+  'flakes/flake-registry.sh',
   'flakes/subdir-flake.sh',
   'gc.sh',
   'nix-collect-garbage-d.sh',