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diff --git a/doc/manual/src/language/advanced-attributes.md b/doc/manual/src/language/advanced-attributes.md new file mode 100644 index 000000000..5a63236e5 --- /dev/null +++ b/doc/manual/src/language/advanced-attributes.md @@ -0,0 +1,332 @@ +# Advanced Attributes + +Derivations can declare some infrequently used optional attributes. + + - [`allowedReferences`]{#adv-attr-allowedReferences}\ + The optional attribute `allowedReferences` specifies a list of legal + references (dependencies) of the output of the builder. For example, + + ```nix + allowedReferences = []; + ``` + + enforces that the output of a derivation cannot have any runtime + dependencies on its inputs. To allow an output to have a runtime + dependency on itself, use `"out"` as a list item. This is used in + NixOS to check that generated files such as initial ramdisks for + booting Linux don’t have accidental dependencies on other paths in + the Nix store. + + - [`allowedRequisites`]{#adv-attr-allowedRequisites}\ + This attribute is similar to `allowedReferences`, but it specifies + the legal requisites of the whole closure, so all the dependencies + recursively. For example, + + ```nix + allowedRequisites = [ foobar ]; + ``` + + enforces that the output of a derivation cannot have any other + runtime dependency than `foobar`, and in addition it enforces that + `foobar` itself doesn't introduce any other dependency itself. + + - [`disallowedReferences`]{#adv-attr-disallowedReferences}\ + The optional attribute `disallowedReferences` specifies a list of + illegal references (dependencies) of the output of the builder. For + example, + + ```nix + disallowedReferences = [ foo ]; + ``` + + enforces that the output of a derivation cannot have a direct + runtime dependencies on the derivation `foo`. + + - [`disallowedRequisites`]{#adv-attr-disallowedRequisites}\ + This attribute is similar to `disallowedReferences`, but it + specifies illegal requisites for the whole closure, so all the + dependencies recursively. For example, + + ```nix + disallowedRequisites = [ foobar ]; + ``` + + enforces that the output of a derivation cannot have any runtime + dependency on `foobar` or any other derivation depending recursively + on `foobar`. + + - [`exportReferencesGraph`]{#adv-attr-exportReferencesGraph}\ + This attribute allows builders access to the references graph of + their inputs. The attribute is a list of inputs in the Nix store + whose references graph the builder needs to know. The value of + this attribute should be a list of pairs `[ name1 path1 name2 + path2 ... ]`. The references graph of each *pathN* will be stored + in a text file *nameN* in the temporary build directory. The text + files have the format used by `nix-store --register-validity` + (with the deriver fields left empty). For example, when the + following derivation is built: + + ```nix + derivation { + ... + exportReferencesGraph = [ "libfoo-graph" libfoo ]; + }; + ``` + + the references graph of `libfoo` is placed in the file + `libfoo-graph` in the temporary build directory. + + `exportReferencesGraph` is useful for builders that want to do + something with the closure of a store path. Examples include the + builders in NixOS that generate the initial ramdisk for booting + Linux (a `cpio` archive containing the closure of the boot script) + and the ISO-9660 image for the installation CD (which is populated + with a Nix store containing the closure of a bootable NixOS + configuration). + + - [`impureEnvVars`]{#adv-attr-impureEnvVars}\ + This attribute allows you to specify a list of environment variables + that should be passed from the environment of the calling user to + the builder. Usually, the environment is cleared completely when the + builder is executed, but with this attribute you can allow specific + environment variables to be passed unmodified. For example, + `fetchurl` in Nixpkgs has the line + + ```nix + impureEnvVars = [ "http_proxy" "https_proxy" ... ]; + ``` + + to make it use the proxy server configuration specified by the user + in the environment variables `http_proxy` and friends. + + This attribute is only allowed in *fixed-output derivations* (see + below), where impurities such as these are okay since (the hash + of) the output is known in advance. It is ignored for all other + derivations. + + > **Warning** + > + > `impureEnvVars` implementation takes environment variables from + > the current builder process. When a daemon is building its + > environmental variables are used. Without the daemon, the + > environmental variables come from the environment of the + > `nix-build`. + + - [`outputHash`]{#adv-attr-outputHash}; [`outputHashAlgo`]{#adv-attr-outputHashAlgo}; [`outputHashMode`]{#adv-attr-outputHashMode}\ + These attributes declare that the derivation is a so-called + *fixed-output derivation*, which means that a cryptographic hash of + the output is already known in advance. When the build of a + fixed-output derivation finishes, Nix computes the cryptographic + hash of the output and compares it to the hash declared with these + attributes. If there is a mismatch, the build fails. + + The rationale for fixed-output derivations is derivations such as + those produced by the `fetchurl` function. This function downloads a + file from a given URL. To ensure that the downloaded file has not + been modified, the caller must also specify a cryptographic hash of + the file. For example, + + ```nix + fetchurl { + url = "http://ftp.gnu.org/pub/gnu/hello/hello-2.1.1.tar.gz"; + sha256 = "1md7jsfd8pa45z73bz1kszpp01yw6x5ljkjk2hx7wl800any6465"; + } + ``` + + It sometimes happens that the URL of the file changes, e.g., because + servers are reorganised or no longer available. We then must update + the call to `fetchurl`, e.g., + + ```nix + fetchurl { + url = "ftp://ftp.nluug.nl/pub/gnu/hello/hello-2.1.1.tar.gz"; + sha256 = "1md7jsfd8pa45z73bz1kszpp01yw6x5ljkjk2hx7wl800any6465"; + } + ``` + + If a `fetchurl` derivation was treated like a normal derivation, the + output paths of the derivation and *all derivations depending on it* + would change. For instance, if we were to change the URL of the + Glibc source distribution in Nixpkgs (a package on which almost all + other packages depend) massive rebuilds would be needed. This is + unfortunate for a change which we know cannot have a real effect as + it propagates upwards through the dependency graph. + + For fixed-output derivations, on the other hand, the name of the + output path only depends on the `outputHash*` and `name` attributes, + while all other attributes are ignored for the purpose of computing + the output path. (The `name` attribute is included because it is + part of the path.) + + As an example, here is the (simplified) Nix expression for + `fetchurl`: + + ```nix + { stdenv, curl }: # The curl program is used for downloading. + + { url, sha256 }: + + stdenv.mkDerivation { + name = baseNameOf (toString url); + builder = ./builder.sh; + buildInputs = [ curl ]; + + # This is a fixed-output derivation; the output must be a regular + # file with SHA256 hash sha256. + outputHashMode = "flat"; + outputHashAlgo = "sha256"; + outputHash = sha256; + + inherit url; + } + ``` + + The `outputHashAlgo` attribute specifies the hash algorithm used to + compute the hash. It can currently be `"sha1"`, `"sha256"` or + `"sha512"`. + + The `outputHashMode` attribute determines how the hash is computed. + It must be one of the following two values: + + - `"flat"`\ + The output must be a non-executable regular file. If it isn’t, + the build fails. The hash is simply computed over the contents + of that file (so it’s equal to what Unix commands like + `sha256sum` or `sha1sum` produce). + + This is the default. + + - `"recursive"`\ + The hash is computed over the NAR archive dump of the output + (i.e., the result of [`nix-store + --dump`](../command-ref/nix-store.md#operation---dump)). In + this case, the output can be anything, including a directory + tree. + + The `outputHash` attribute, finally, must be a string containing + the hash in either hexadecimal or base-32 notation. (See the + [`nix-hash` command](../command-ref/nix-hash.md) for information + about converting to and from base-32 notation.) + + - [`__contentAddressed`]{#adv-attr-__contentAddressed} + If this **experimental** attribute is set to true, then the derivation + outputs will be stored in a content-addressed location rather than the + traditional input-addressed one. + This only has an effect if the `ca-derivations` experimental feature is enabled. + + Setting this attribute also requires setting `outputHashMode` and `outputHashAlgo` like for *fixed-output derivations* (see above). + + - [`passAsFile`]{#adv-attr-passAsFile}\ + A list of names of attributes that should be passed via files rather + than environment variables. For example, if you have + + ```nix + passAsFile = ["big"]; + big = "a very long string"; + ``` + + then when the builder runs, the environment variable `bigPath` + will contain the absolute path to a temporary file containing `a + very long string`. That is, for any attribute *x* listed in + `passAsFile`, Nix will pass an environment variable `xPath` + holding the path of the file containing the value of attribute + *x*. This is useful when you need to pass large strings to a + builder, since most operating systems impose a limit on the size + of the environment (typically, a few hundred kilobyte). + + - [`preferLocalBuild`]{#adv-attr-preferLocalBuild}\ + If this attribute is set to `true` and [distributed building is + enabled](../advanced-topics/distributed-builds.md), then, if + possible, the derivation will be built locally instead of forwarded + to a remote machine. This is appropriate for trivial builders + where the cost of doing a download or remote build would exceed + the cost of building locally. + + - [`allowSubstitutes`]{#adv-attr-allowSubstitutes}\ + If this attribute is set to `false`, then Nix will always build this + derivation; it will not try to substitute its outputs. This is + useful for very trivial derivations (such as `writeText` in Nixpkgs) + that are cheaper to build than to substitute from a binary cache. + + > **Note** + > + > You need to have a builder configured which satisfies the + > derivation’s `system` attribute, since the derivation cannot be + > substituted. Thus it is usually a good idea to align `system` with + > `builtins.currentSystem` when setting `allowSubstitutes` to + > `false`. For most trivial derivations this should be the case. + + - [`__structuredAttrs`]{#adv-attr-structuredAttrs}\ + If the special attribute `__structuredAttrs` is set to `true`, the other derivation + attributes are serialised in JSON format and made available to the + builder via the file `.attrs.json` in the builder’s temporary + directory. This obviates the need for [`passAsFile`](#adv-attr-passAsFile) since JSON files + have no size restrictions, unlike process environments. + + It also makes it possible to tweak derivation settings in a structured way; see + [`outputChecks`](#adv-attr-outputChecks) for example. + + As a convenience to Bash builders, + Nix writes a script named `.attrs.sh` to the builder’s directory + that initialises shell variables corresponding to all attributes + that are representable in Bash. This includes non-nested + (associative) arrays. For example, the attribute `hardening.format = true` + ends up as the Bash associative array element `${hardening[format]}`. + + - [`outputChecks`]{#adv-attr-outputChecks}\ + When using [structured attributes](#adv-attr-structuredAttrs), the `outputChecks` + attribute allows defining checks per-output. + + In addition to + [`allowedReferences`](#adv-attr-allowedReferences), [`allowedRequisites`](#adv-attr-allowedRequisites), + [`disallowedReferences`](#adv-attr-disallowedReferences) and [`disallowedRequisites`](#adv-attr-disallowedRequisites), + the following attributes are available: + + - `maxSize` defines the maximum size of the resulting [store object](../glossary.md#gloss-store-object). + - `maxClosureSize` defines the maximum size of the output's closure. + - `ignoreSelfRefs` controls whether self-references should be considered when + checking for allowed references/requisites. + + Example: + + ```nix + __structuredAttrs = true; + + outputChecks.out = { + # The closure of 'out' must not be larger than 256 MiB. + maxClosureSize = 256 * 1024 * 1024; + + # It must not refer to the C compiler or to the 'dev' output. + disallowedRequisites = [ stdenv.cc "dev" ]; + }; + + outputChecks.dev = { + # The 'dev' output must not be larger than 128 KiB. + maxSize = 128 * 1024; + }; + ``` + + - [`unsafeDiscardReferences`]{#adv-attr-unsafeDiscardReferences}\ + > **Warning** + > This is an experimental feature. + > + > To enable it, add the following to [nix.conf](../command-ref/conf-file.md): + > + > ``` + > extra-experimental-features = discard-references + > ``` + + When using [structured attributes](#adv-attr-structuredAttrs), the + attribute `unsafeDiscardReferences` is an attribute set with a boolean value for each output name. + If set to `true`, it disables scanning the output for runtime dependencies. + + Example: + + ```nix + __structuredAttrs = true; + unsafeDiscardReferences.out = true; + ``` + + This is useful, for example, when generating self-contained filesystem images with + their own embedded Nix store: hashes found inside such an image refer + to the embedded store and not to the host's Nix store. |