nix repl: add :log

Add a :log command that shows logs for a derivation.

Closes https://github.com/NixOS/nix/issues/3504

Co-authored-by: Taeer Bar-Yam <taeer@bar-yam.me>

2 files changed, 29 insertions, 4 deletions

diff --git a/src/nix/repl.cc b/src/nix/repl.cc
index 2649eb0bd..42143871f 100644
--- a/src/nix/repl.cc
+++ b/src/nix/repl.cc
@@ -430,7 +430,8 @@ bool NixRepl::processLine(string line)
              << "  :s <expr>     Build dependencies of derivation, then start nix-shell\n"
              << "  :t <expr>     Describe result of evaluation\n"
              << "  :u <expr>     Build derivation, then start nix-shell\n"
-             << "  :doc <expr>   Show documentation of a builtin function\n";
+             << "  :doc <expr>   Show documentation of a builtin function\n"
+             << "  :log <expr>   Show logs for a derivation\n";
     }
 
     else if (command == ":a" || command == ":add") {
@@ -500,7 +501,7 @@ bool NixRepl::processLine(string line)
         runNix("nix-shell", {state->store->printStorePath(drvPath)});
     }
 
-    else if (command == ":b" || command == ":i" || command == ":s") {
+    else if (command == ":b" || command == ":i" || command == ":s" || command == ":log") {
         Value v;
         evalString(arg, v);
         StorePath drvPath = getDerivationPath(v);
@@ -514,6 +515,27 @@ bool NixRepl::processLine(string line)
                 logger->cout("  %s -> %s", outputName, state->store->printStorePath(outputPath));
         } else if (command == ":i") {
             runNix("nix-env", {"-i", drvPathRaw});
+        } else if (command == ":log") {
+            settings.readOnlyMode = true;
+            Finally roModeReset([&]() {
+                settings.readOnlyMode = false;
+            });
+            auto subs = getDefaultSubstituters();
+
+            subs.push_front(state->store);
+
+            bool foundLog = false;
+            RunPager pager;
+            for (auto & sub : subs) {
+                auto log = sub->getBuildLog(drvPath);
+                if (log) {
+                    printInfo("got build log for '%s' from '%s'", drvPathRaw, sub->getUri());
+                    logger->writeToStdout(*log);
+                    foundLog = true;
+                    break;
+                }
+            }
+            if (!foundLog) throw Error("build log of '%s' is not available", drvPathRaw);
         } else {
             runNix("nix-shell", {drvPathRaw});
         }
diff --git a/src/nix/repl.md b/src/nix/repl.md
index bba60f871..9b6f2bee3 100644
--- a/src/nix/repl.md
+++ b/src/nix/repl.md
@@ -35,14 +35,17 @@ R""(
   nix-repl> emacs.drvPath
   "/nix/store/lp0sjrhgg03y2n0l10n70rg0k7hhyz0l-emacs-27.1.drv"
 
-  nix-repl> drv = runCommand "hello" { buildInputs = [ hello ]; } "hello > $out"
+  nix-repl> drv = runCommand "hello" { buildInputs = [ hello ]; } "hello; hello > $out"
 
-  nix-repl> :b x
+  nix-repl> :b drv
   this derivation produced the following outputs:
     out -> /nix/store/0njwbgwmkwls0w5dv9mpc1pq5fj39q0l-hello
 
   nix-repl> builtins.readFile drv
   "Hello, world!\n"
+
+  nix-repl> :log drv
+  Hello, world!
   ```
 
 # Description