nix-super/doc/manual/source/protocols/store-path.md

Complete Store Path Calculation

This is the complete specification for how [store path]s are calculated.

The format of this specification is close to Extended Backus–Naur form, but must deviate for a few things such as hash functions which we treat as bidirectional for specification purposes.

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.

store path

Store path proper

store-path = store-dir "/" digest "-" name

where

• name = the name of the store object.

• store-dir = the store directory

• digest = base-32 representation of the first 160 bits of a SHA-256 hash of fingerprint

  This the hash part of the store name

Fingerprint

• fingerprint = 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).

• type = one of:

  • | "text" { ":" store-path }
    

    This is for the "Text" method of content addressing store objects. The optional trailing store paths are the references of the store object.

  • | "source" { ":" store-path } [ ":self" ]
    

    This is for the "Nix Archive" method of content addressing store objects, if the hash algorithm is 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.

  • | "output:" id
    

    For either the outputs built from derivations, or content-addressed store objects that are not using one of the two above cases. To be explicit about the latter, that is currently these methods:

    • "Flat"
    • "Git"
    • "Nix Archive" if the hash algorithm is not SHA-256.

    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-fingerprint

Inner fingerprint

• inner-fingerprint = one of the following based on type:

  • if type = "text:" ...:

    the string written to the resulting store path.

  • if type = "source:" ...:

    the hash of the Nix Archive (NAR) serialization of the file system object of the store object.

  • if type = "output:" id:

    • For input-addressed derivation outputs:

      the ATerm serialization of the derivation modulo fixed output derivations.

    • For content-addressed store paths:

      "fixed:out:" rec algo ":" hash ":"
      

      where

      • rec = one of:

        • | ""
          

          (empty string) for hashes of the flat (single file) serialization

        • | "r:"
          

          hashes of the for Nix Archive (NAR) (arbitrary file system object) serialization

        • | "git:"
          

          hashes of the Git blob/tree Merkel tree format

      • algo = "md5" | "sha1" | "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.

Historical Note

The type = "source:" ... and type = "output:out" grammars technically overlap in purpose, 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-fingerprint starting with output:out:, while the latter would have an inner-fingerprint starting with source:.

Since 64519cfd657d024ae6e2bb74cb21ad21b886fd2a (2008), however, it was decided that separating derivation-produced vs manually-hashed content-addressed data like this was not useful. Now, data that is content-addressed with SHA-256 + NAR-serialization always uses the source:... construction, regardless of how it was produced (manually or by derivation). This allows freely switching between using fixed-output derivations for fetching, and fetching out-of-band and then manually adding. It also removes the ambiguity from the grammar.