Include store path exact spec in the docs

I think this isn't quite ready until some other things are documented
fist, like NAR.

doc/manual/src/SUMMARY.md.in

@@ -20,6 +20,7 @@
   - [File System Object](store/file-system-object.md)
   - [Store Object](store/store-object.md)
   - [Store Path](store/store-path.md)
+    - [Exact Store Path Specification](store/store-path/exact-spec.md)
   - [Store Types](store/types/index.md)
 {{#include ./store/types/SUMMARY.md}}
 - [Nix Language](language/index.md)

doc/manual/src/store/store-path/exact-spec.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](../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](../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 +65,11 @@ StorePath Store::followLinksToStorePath(std::string_view path) const
 }
 
 
-/* Store paths have the following form:
-
-   <realized-path> = <store>/<h>-<name>
-
-   where
-
-   <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:".
+/*
+The algorithm here is documented in the Store section of the Nix
+manual.
+
+Please keep that documentation up to date with any changes made below.
 */