Add 'nix why-depends' manpage

src/nix/why-depends.cc

@@ -50,22 +50,11 @@ struct CmdWhyDepends : SourceExprCommand
         return "show why a package has another package in its closure";
     }
 
-    Examples examples() override
+    std::string doc() override
     {
-        return {
+        return
-            Example{
+          #include "why-depends.md"
-                "To show one path through the dependency graph leading from Hello to Glibc:",
+          ;
-                "nix why-depends nixpkgs#hello nixpkgs#glibc"
-            },
-            Example{
-                "To show all files and paths in the dependency graph leading from Thunderbird to libX11:",
-                "nix why-depends --all nixpkgs#thunderbird nixpkgs#xorg.libX11"
-            },
-            Example{
-                "To show why Glibc depends on itself:",
-                "nix why-depends nixpkgs#glibc nixpkgs#glibc"
-            },
-        };
     }
 
     Category category() override { return catSecondary; }

src/nix/why-depends.md

@@ -0,0 +1,80 @@
+R""(
+
+# Examples
+
+* Show one path through the dependency graph leading from Hello to
+  Glibc:
+
+  ```console
+  # nix why-depends nixpkgs#hello nixpkgs#glibc
+  /nix/store/v5sv61sszx301i0x6xysaqzla09nksnd-hello-2.10
+  └───bin/hello: …...................../nix/store/9l06v7fc38c1x3r2iydl15ksgz0ysb82-glibc-2.32/lib/ld-linux-x86-64.…
+      → /nix/store/9l06v7fc38c1x3r2iydl15ksgz0ysb82-glibc-2.32
+  ```
+
+* Show all files and paths in the dependency graph leading from
+  Thunderbird to libX11:
+
+  ```console
+  # nix why-depends --all nixpkgs#thunderbird nixpkgs#xorg.libX11
+  /nix/store/qfc8729nzpdln1h0hvi1ziclsl3m84sr-thunderbird-78.5.1
+  ├───lib/thunderbird/libxul.so: …6wrw-libxcb-1.14/lib:/nix/store/adzfjjh8w25vdr0xdx9x16ah4f5rqrw5-libX11-1.7.0/lib:/nix/store/ssf…
+  │   → /nix/store/adzfjjh8w25vdr0xdx9x16ah4f5rqrw5-libX11-1.7.0
+  ├───lib/thunderbird/libxul.so: …pxyc-libXt-1.2.0/lib:/nix/store/1qj29ipxl2fyi2b13l39hdircq17gnk0-libXdamage-1.1.5/lib:/nix/store…
+  │   → /nix/store/1qj29ipxl2fyi2b13l39hdircq17gnk0-libXdamage-1.1.5
+  │   ├───lib/libXdamage.so.1.1.0: …-libXfixes-5.0.3/lib:/nix/store/adzfjjh8w25vdr0xdx9x16ah4f5rqrw5-libX11-1.7.0/lib:/nix/store/9l0…
+  │   │   → /nix/store/adzfjjh8w25vdr0xdx9x16ah4f5rqrw5-libX11-1.7.0
+  …
+  ```
+
+* Show why Glibc depends on itself:
+
+  ```console
+  # nix why-depends nixpkgs#glibc nixpkgs#glibc
+  /nix/store/9df65igwjmf2wbw0gbrrgair6piqjgmi-glibc-2.31
+  └───lib/ld-2.31.so: …che       Do not use /nix/store/9df65igwjmf2wbw0gbrrgair6piqjgmi-glibc-2.31/etc/ld.so.cache.  --…
+      → /nix/store/9df65igwjmf2wbw0gbrrgair6piqjgmi-glibc-2.31
+  ```
+
+* Show why Geeqie has a build-time dependency on `systemd`:
+
+  ```console
+  # nix why-depends --derivation nixpkgs#geeqie nixpkgs#systemd
+  /nix/store/drrpq2fqlrbj98bmazrnww7hm1in3wgj-geeqie-1.4.drv
+  └───/: …atch.drv",["out"]),("/nix/store/qzh8dyq3lfbk3i1acbp7x9wh3il2imiv-gtk+3-3.24.21.drv",["dev"]),("/…
+      → /nix/store/qzh8dyq3lfbk3i1acbp7x9wh3il2imiv-gtk+3-3.24.21.drv
+      └───/: …16.0.drv",["dev"]),("/nix/store/8kp79fyslf3z4m3dpvlh6w46iaadz5c2-cups-2.3.3.drv",["dev"]),("/nix…
+          → /nix/store/8kp79fyslf3z4m3dpvlh6w46iaadz5c2-cups-2.3.3.drv
+          └───/: ….3.1.drv",["out"]),("/nix/store/yd3ihapyi5wbz1kjacq9dbkaq5v5hqjg-systemd-246.4.drv",["dev"]),("/…
+              → /nix/store/yd3ihapyi5wbz1kjacq9dbkaq5v5hqjg-systemd-246.4.drv
+  ```
+
+# Description
+
+Nix automatically determines potential runtime dependencies between
+store paths by scanning for the *hash parts* of store paths. For
+instance, if there exists a store path
+`/nix/store/9df65igwjmf2wbw0gbrrgair6piqjgmi-glibc-2.31`, and a file
+inside another store path contains the string `9df65igw…`, then the
+latter store path *refers* to the former, and thus might need it at
+runtime. Nix always maintains the existence of the transitive closure
+of a store path under the references relationship; it is therefore not
+possible to install a store path without having all of its references
+present.
+
+Sometimes Nix packages end up with unexpected runtime dependencies;
+for instance, a reference to a compiler might accidentally end up in a
+binary, causing the former to be in the latter's closure. This kind of
+*closure size bloat* is undesirable.
+
+`nix why-depends` allows you to diagnose the cause of such issues. It
+shows why the store path *package* depends on the store path
+*dependency*, by showing a shortest sequence in the references graph
+from the former to the latter. Also, for each node along this path, it
+shows a file fragment containing a reference to the next store path in
+the sequence.
+
+To show why derivation *package* has a build-time rather than runtime
+dependency on derivation *dependency*, use `--derivation`.
+
+)""