document store paths

update the glossary to point to the new page.

since this is a cross-cutting concern, it warrants its own section in
the manual.

Co-authored-by: John Ericson <git@JohnEricson.me>

doc/manual/src/SUMMARY.md.in

@@ -19,6 +19,7 @@
 - [Nix Store](store/index.md)
   - [File System Object](store/file-system-object.md)
   - [Store Object](store/store-object.md)
+  - [Store Path](store/store-path.md)
 - [Nix Language](language/index.md)
   - [Data Types](language/values.md)
   - [Language Constructs](language/constructs.md)

doc/manual/src/glossary.md

@@ -86,10 +86,13 @@
 
 - [store path]{#gloss-store-path}
 
-  The location of a [store object] in the file system, i.e., an
+  The location of a [store object](@docroot@/store/index.md#store-object) in the file system, i.e., an immediate child of the Nix store directory.
-  immediate child of the Nix store directory.
 
-  Example: `/nix/store/a040m110amc4h71lds2jmr8qrkj2jhxd-git-2.38.1`
+  > **Example**
+  >
+  > `/nix/store/a040m110amc4h71lds2jmr8qrkj2jhxd-git-2.38.1`
+
+  See [Store Path](@docroot@/store/store-path.md) for details.
 
   [store path]: #gloss-store-path
 

doc/manual/src/store/store-object.md

@@ -4,7 +4,7 @@ A Nix store is a collection of *store objects* with *references* between them.
 A store object consists of
 
   - A [file system object](./file-system-object.md) as data
-  - A set of [store paths](@docroot@/glossary.md#gloss-store-path) as references to other store objects
+  - A set of [store paths](./store-path.md) as references to other store objects
 
 Store objects are [immutable](https://en.wikipedia.org/wiki/Immutable_object):
 Once created, they do not change until they are deleted.

doc/manual/src/store/store-path.md

@@ -0,0 +1,69 @@
+# Store Path
+
+Nix implements references to [store objects](./index.md#store-object) as *store paths*.
+
+Think of a store path as an [opaque], [unique identifier]:
+The only way to obtain store path is by adding or building store objects.
+A store path will always reference exactly one store object.
+
+[opaque]: https://en.m.wikipedia.org/wiki/Opaque_data_type
+[unique identifier]: https://en.m.wikipedia.org/wiki/Unique_identifier
+
+Store paths are pairs of
+
+- A 20-byte digest for identification
+- A symbolic name for people to read
+
+> **Example**
+>
+> - Digest: `b6gvzjyb2pg0kjfwrjmg1vfhh54ad73z`
+> - Name:   `firefox-33.1`
+
+To make store objects accessible to operating system processes, stores have to expose store objects through the file system.
+
+A store path is rendered to a file system path as the concatenation of
+
+- [Store directory](#store-directory) (typically `/nix/store`)
+- Path separator (`/`)
+- Digest rendered in a custom variant of [Base32](https://en.wikipedia.org/wiki/Base32) (20 arbitrary bytes become 32 ASCII characters)
+- Hyphen (`-`)
+- Name
+
+> **Example**
+>
+> ```
+>   /nix/store/b6gvzjyb2pg0kjfwrjmg1vfhh54ad73z-firefox-33.1
+>   |--------| |------------------------------| |----------|
+> store directory            digest                 name
+> ```
+
+## Store Directory
+
+Every [Nix store](./index.md) has a store directory.
+
+Not every store can be accessed through the file system.
+But if the store has a file system representation, the store directory contains the store’s [file system objects], which can be addressed by [store paths](#store-path).
+
+[file system objects]: ./file-system-object.md
+
+This means a store path is not just derived from the referenced store object itself, but depends on the store the store object is in.
+
+> **Note**
+>
+> The store directory defaults to `/nix/store`, but is in principle arbitrary.
+
+It is important which store a given store object belongs to:
+Files in the store object can contain store paths, and processes may read these paths.
+Nix can only guarantee referential integrity if store paths do not cross store boundaries.
+
+Therefore one can only copy store objects to a different store if
+
+- The source and target stores' directories match
+
+  or
+
+- The store object in question has no references, that is, contains no store paths
+
+One cannot copy a store object to a store with a different store directory.
+Instead, it has to be rebuilt, together with all its dependencies.
+It is in general not enough to replace the store directory string in file contents, as this may render executables unusable by invalidating their internal offsets or checksums.