Include store path exact spec in the docs

This is niche, but deserves to be in the manual because it is describing
behavior visible to the outside world, not mere implementation details.

doc/manual/src/SUMMARY.md.in

@@ -106,6 +106,7 @@
 - [Architecture and Design](architecture/architecture.md)
 - [Protocols](protocols/index.md)
   - [Serving Tarball Flakes](protocols/tarball-fetcher.md)
+  - [Exact Store Path Specification](protocols/store-path.md)
   - [Derivation "ATerm" file format](protocols/derivation-aterm.md)
 - [Glossary](glossary.md)
 - [Contributing](contributing/index.md)

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

@@ -0,0 +1,104 @@
+# Complete Store Path Calculation
+
+This is the complete specification for how store paths are calculated.
+
+Regular users do *not* need to know this information --- store paths can be treated as black boxes computed from the properties of the store objects they refer to.
+But for those interested in exactly how Nix works, e.g. if they are reimplementing it, this information can be useful.
+
+```bnf
+<realized-path> ::= <store-dir>/<digest>-<name>
+```
+where
+
+- `<digest>` = base-32 representation of the first 160 bits of a [SHA-256] hash of `<pre>`
+
+  Th is :the hash part of the store name
+
+- `<pre>` = the string `<type>:sha256:<inner-digest>:<store>:<name>`;
+
+  Note that it includes the location of the store as well as the name to make sure that changes to either of those are reflected in the hash
+  (e.g. you won't get `/nix/store/<digest>-name1` and `/nix/store/<digest>-name2`, or `/gnu/store/<digest>-name1`, with equal hash parts).
+
+- `<name>` = the name of the store object.
+
+- `<store>` = the [store directory](@docroot@/store/store-path.md#store-directory)
+
+- `<type>` = one of:
+
+  - ```bnf
+    text:<r1>:<r2>:...<rN>
+    ```
+
+    for encoded derivations written to the store.
+    `<r1> ... <rN>` are the store paths referenced by this path.
+    Those are encoded in the form described by `<realized-path>`.
+
+  - ```bnf
+    source:<r1>:<r2>:...:<rN>:self
+    ```
+
+    For paths copied to the store and hashed via a [Nix Archive (NAR)] and [SHA-256][sha-256].
+    Just like in the text case, we can have the store objects referenced by their paths.
+    Additionally, we can have an optional `:self` label to denote self reference.
+
+  - ```bnf
+    output:<id>
+    ```
+
+    For either the outputs built from derivations,
+    paths copied to the store hashed that area single file hashed directly, or the via a hash algorithm other than [SHA-256][sha-256].
+    (in that case "source" is used; it's silly, but it's done that way for compatibility).
+
+    `<id>` is the name of the output (usually, "out").
+    For content-addressed store objects, `<id>`, is always "out".
+
+- `<inner-digest>` = base-16 representation of a SHA-256 hash of `<inner-pre>`
+
+- `<inner-pre>` = one of the following based on `<type>`:
+
+  - if `<type>` = `text:...`:
+
+    the string written to the resulting store path.
+
+  - if `<type>` = `source:...`:
+
+    the the hash of the [Nix Archive (NAR)] serialization of the [file system object](@docroot@/store/file-system-object.md) of the store object.
+
+  - if `<type>` = `output:<id>`:
+
+    - For input-addressed derivation outputs:
+
+      the [ATerm](@docroot@/protocols/derivation-aterm.md) serialization of the derivation modulo fixed output derivations.
+
+    - For content-addressed store paths:
+
+      the string `fixed:out:<rec><algo>:<hash>:`, where
+
+      - `<rec>` = one of:
+
+        - `r:` hashes of the for [Nix Archive (NAR)] (arbitrary file system object) serialization
+
+        - `` (empty string) for hashes of the flat (single file) serialization
+
+      - `<algo>` = `md5`, `sha1` or `sha256`
+
+      - `<hash>` = base-16 representation of the path or flat hash of the contents of the path (or expected contents of the path for fixed-output derivations).
+
+      Note that `<id>` = `out`, regardless of the name part of the store path.
+      Also note that NAR + SHA-256 must not use this case, and instead must use the `<type>` = `source:...` case.
+
+[Nix Archive (NAR)]: @docroot@/glossary.md#gloss-NAR
+[sha-256]: https://en.m.wikipedia.org/wiki/SHA-256
+
+## Historical Note
+
+The `<type>` = `source:...` and `<type>` = `output:out` grammars technically overlap, in that both can represent data hashed by its SHA-256 NAR serialization.
+
+The original reason for this way of computing names was to prevent name collisions (for security).
+For instance, the thinking was that it shouldn't be feasible to come up with a derivation whose output path collides with the path for a copied source.
+The former would have an `<inner-pre>` starting with `output:out:`, while the latter would have an `<inner-pre>` starting with `source:`.
+
+Since `64519cfd657d024ae6e2bb74cb21ad21b886fd2a` (2008), however, it was decided that separting derivation-produced vs manually-hashed content-addressed data like this was not useful.
+Now, data this is to be SHA-256 + NAR-serialization content-addressed always uses the `source:...` construction, regardless of how it was produced (manually or by derivation).
+This allows freely switching between using [fixed-output derivations](@docroot@/glossary.md#gloss-fixed-output-derivation) for fetching, and fetching out-of-band and then manually adding.
+It also removes the ambiguity from the grammar.

src/libstore/store-api.cc

@@ -65,85 +65,13 @@ StorePath Store::followLinksToStorePath(std::string_view path) const
 }
 
 
-/* Store paths have the following form:
+/*
+The exact specification of store paths is in `protocols/store-path.md`
+in the Nix manual. These few functions implement that specification.
 
-   <realized-path> = <store>/<h>-<name>
+If changes do these functions go behind mere implementation changes but
-
+also update the user-visible behavior, please update the specification
-   where
+to match.
-
-   <store> = the location of the Nix store, usually /nix/store
-
-   <name> = a human readable name for the path, typically obtained
-     from the name attribute of the derivation, or the name of the
-     source file from which the store path is created.  For derivation
-     outputs other than the default "out" output, the string "-<id>"
-     is suffixed to <name>.
-
-   <h> = base-32 representation of the first 160 bits of a SHA-256
-     hash of <s>; the hash part of the store name
-
-   <s> = the string "<type>:sha256:<h2>:<store>:<name>";
-     note that it includes the location of the store as well as the
-     name to make sure that changes to either of those are reflected
-     in the hash (e.g. you won't get /nix/store/<h>-name1 and
-     /nix/store/<h>-name2 with equal hash parts).
-
-   <type> = one of:
-     "text:<r1>:<r2>:...<rN>"
-       for plain text files written to the store using
-       addTextToStore(); <r1> ... <rN> are the store paths referenced
-       by this path, in the form described by <realized-path>
-     "source:<r1>:<r2>:...:<rN>:self"
-       for paths copied to the store using addToStore() when recursive
-       = true and hashAlgo = "sha256". Just like in the text case, we
-       can have the store paths referenced by the path.
-       Additionally, we can have an optional :self label to denote self
-       reference.
-     "output:<id>"
-       for either the outputs created by derivations, OR paths copied
-       to the store using addToStore() with recursive != true or
-       hashAlgo != "sha256" (in that case "source" is used; it's
-       silly, but it's done that way for compatibility).  <id> is the
-       name of the output (usually, "out").
-
-   <h2> = base-16 representation of a SHA-256 hash of <s2>
-
-   <s2> =
-     if <type> = "text:...":
-       the string written to the resulting store path
-     if <type> = "source:...":
-       the serialisation of the path from which this store path is
-       copied, as returned by hashPath()
-     if <type> = "output:<id>":
-       for non-fixed derivation outputs:
-         the derivation (see hashDerivationModulo() in
-         primops.cc)
-       for paths copied by addToStore() or produced by fixed-output
-       derivations:
-         the string "fixed:out:<rec><algo>:<hash>:", where
-           <rec> = "r:" for recursive (path) hashes, or "" for flat
-             (file) hashes
-           <algo> = "md5", "sha1" or "sha256"
-           <hash> = base-16 representation of the path or flat hash of
-             the contents of the path (or expected contents of the
-             path for fixed-output derivations)
-
-   Note that since an output derivation has always type output, while
-   something added by addToStore can have type output or source depending
-   on the hash, this means that the same input can be hashed differently
-   if added to the store via addToStore or via a derivation, in the sha256
-   recursive case.
-
-   It would have been nicer to handle fixed-output derivations under
-   "source", e.g. have something like "source:<rec><algo>", but we're
-   stuck with this for now...
-
-   The main reason for this way of computing names is to prevent name
-   collisions (for security).  For instance, it shouldn't be feasible
-   to come up with a derivation whose output path collides with the
-   path for a copied source.  The former would have a <s> starting with
-   "output:out:", while the latter would have a <s> starting with
-   "source:".
 */