Yocto Mirrors Mechanism

As a part of my work for Inango Systems I investigated BitBake support for mirrors (and even provided bugfix as one of the results of this investigation) and prepared this article.


The BitBake build utility, adapted by the Yocto Project, allows for flexible configuration of source mirrors for all the remote files required during the build of Yocto distribution. This functionality allows to speed up the distribution build, as well as organize a local backup with sources of all the packages used.

However, this mechanism is rather poorly documented, and we are going to fix this situation in the current article.

BitBake Source Mirrors

The BitBake mirrors mechanism is supported at least in three places:

  1. PREMIRRORS are primary mirrors that will be used before an upstream URI from SRC_URI. It can be used to provide a quick mirrors to speed-up sources downloading.
  2. MIRRORS are additional mirrors that are used when an upstream URI from SRC_URI is not accessible. Creating MIRRORS is a good idea for a long-living distributions, when a distribution can out-live upstream sources of a used software.
  3. SSTATE_MIRRORS are mirrors for prebuilt cache data objects. It can be used to share pre-built objects from CI builds.

The Yocto Project additionally provide the support for the next mechanisms:

  1. own-mirrors.bbclass is a standard way to provide a single pre-mirror for all supported fetchers:

     INHERIT += "own-mirrors"
     SOURCE_MIRROR_URL = "http://example.com/my-source-mirror"
  2. BB_GENERATE_MIRROR_TARBALLS variable can be defined to “1” to allow reusing DL_DIR from the current build as a mirror for other builds. Without a BB_GENERATE_MIRROR_TARBALLS variable resulting DL_DIR would not provide suitable mirrors for sources fetched from a VCS repositories.
  3. BB_FETCH_PREMIRRORONLY and SSTATE_MIRROR_ALLOW_NETWORK variables can be used to configure BitBake to download sources and build artifacts only from configured mirrors. This variables will be usable for all BB_NO_NETWORK configurations.

Yocto Documentation on Mirrors Syntax

All things described in the previos part are correctly documented and can be found in the official documentation. But a syntax for mirror rules itself is poorly documented. An official documentation suggests only three examples how mirror rules can be created:

  1. In the example[1] for a PREMIRRORS variable we can see that we should use .*/.* regexp to match all URI for a specific fetcher:

     PREMIRRORS_prepend = "\
     git://.*/.* http://www.yoctoproject.org/sources/ \n \
     ftp://.*/.* http://www.yoctoproject.org/sources/ \n \
     http://.*/.* http://www.yoctoproject.org/sources/ \n \
     https://.*/.* http://www.yoctoproject.org/sources/ \n"
  2. In the example[2] for a SSTATE_MIRRORS variable we can found that there are also supported PATH string in matching:

     file://.* http://someserver.tld/share/sstate/PATH;downloadfilename=PATH \n \
     file://.* file:///some-local-dir/sstate/PATH"
  3. In same SSTATE_MIRROS example[2] we can additionally find information that a mirror syntax support more sophisticated regex usage:

     SSTATE_MIRRORS ?= "file://universal-4.9/(.*) http://server_url_sstate_path/universal-4.8/\1 \n"

But there is no full description of the mirror matching mechanism. We offer the following as a way of filling this gap.

Yocto Mirror Syntax

All mirrors are defined as a “pattern replacement” pair. The BitBake attempts to match a source URI against each “pattern” and on a successfull match the BitBake generates a new mirror URI based on an original URI, by using the “pattern” and “replacement” definitions.

URI Matching

Each URI, including “pattern” and “replacement” strings, is split into six components before matching as follows:


In an upstream URI this parts understood as strings, in a “pattern” this parts is interpreted as python re regexps (parameters is only field that is not regexp), in a “replacement” this parts is interpreted as regexp replacements in terms of python re.sub function.

All parts from an upstream URI matched against regexps from the corresponding part of a “pattern” with the next additional rules:

  1. Bitbake always adds a $ sign to the end of a scheme regex. It was made to be sure that “http” scheme whould not match “https” string.
  2. If a scheme is a “file” then the hostname is assumed to be empty. Otherwise the hostname is a text between “://” and next “/” character.
  3. parameters interpreted as a list of “param=value” pairs. For each such pair specified in “pattern” there are should be a full match in an original SRC_URI. It is the only field that matched with a string equality instead of a regexp matching.

    NOTE: A parameters matching was broken up to Yocto 2.6 release. This issue was found and fixed as a part of work on this documentation.

URI Replacement

If a URI was successfully matched, then bitbake creates a new mirror URI from an upstream URI using a “replacement” string. The BitBake uses next rules to make a replacement:

  1. A replacement is made for each part of the URI separately.
  2. Before replacement is performed, we replace the next substrings in a correspondend part of a “replacement” string:
    1. “TYPE” – this substring in a “replacement” string will be replaced with an original URI scheme.
    2. “HOST” – will be replaced with an original URI hostname.
    3. “PATH” – will be replaced with an original URI path.
    4. “BASENAME” – will be replaced with a basename of “PATH” (with a substring that can be found after the last "/" symbol).
    5. “MIRRORNAME” – will be replaced with a special string obtained as concatenation of a “HOST” with ":" symbols replaced with "." symbols and a “PATH” with "/" and "*" symbols replaced with "." symbols.

    NOTE: In the parameters list this replacement is made only in a value part of a param=value pairs.

  3. Each part except the parameters is replaced with re.sub(part_from_pattern, part_from_replacement, part_from_original_uri, 1) Python command. This means that the BitBake will replace only a first match from a current part and that you can use a Python regex replacement syntax including "\1" syntax to insert parts of a matched URI to a result.
  4. If an original URI scheme differs from a “replacement” scheme then an original parameters would be wiped.
  5. The parameters from a “replacement” should be added to a resulting URI. If there were parameters with the same names in an original URI then a value for this parameters will be overridden.
  6. Finally, the BitBake checks that a path part upon replacement is finished with a suggested “basename”. If a resulting path finished with any other string then a resulting path will be concatenated with a "/basename" string. A suggested “basename” is created according to the next rules:
    1. If scheme was not changed then “basename” is just a basename of path from an originial URI.
    2. If scheme was changed and an original URI is pointed to a single file or a tarball with sources then the same basename as in the first rule will be used.
    3. If scheme was changed and an original URI pointed to a some kind of repository then a mirrortarball name will be used. This name is a fetcher specific. e.g. for a git repositories a tarball name will be “git2_hostname.path.to.repo.git.tar.gz”.

NOTE: Last step means that it is impossible to use the "file:///some/path/b.tar.gz" as a mirror path for the "http://other/path/a.tar.gz", but you still can use the "file:///some/path/a.tar.gz" or the "file:///some/path/prefix_a.tar.gz".

Step by Step Process Description

Let’s check how next example will be proceeded:

SRC_URI = "git://git.invalid.infradead.org/foo/mtd-utils.git;branch=master;tag=1234567890123456789012345678901234567890"
PREMIRRORS = "git://.*/.*;branch=master git://somewhere.org/somedir/MIRRORNAME;protocol=http;branch=master_backup \n"

First of all, we’ll split all the three URI’s into the next parts:

    scheme     = "git"
    user       = ""
    password   = ""
    host       = "git.invalid.infradead.org"
    path       = "/foo/mtd-utils.git"
    parameters = {"branch": "master", "tag": "1234567890123456789012345678901234567890"}
    scheme     = "git$"
    user       = ""
    password   = ""
    host       = ".*"
    path       = "/.*"
    parameters = {"branch": "master"}
    scheme     = "git"
    user       = ""
    password   = ""
    host       = "somewhere.org"
    path       = "/somedir/MIRRORNAME"
    parameters = {"branch": "master_backup", "protocol": "http"}

On the next step the BitBake will match the “upstream” URI parts against the corresponding parts of the “pattern” with a re.match(pattern.part, upstream.part) command for all the parts except the “parameters”. For the “parameters” the BitBake will check that the value for a “branch” parameter in the “upstrem” URI and the “pattern” URI are equal.

When these checks pass, the BitBake will start a replacement process. In each part of the “replacement” the BitBake will make the replacements for the special strings:

    scheme     = "git"
    user       = ""
    password   = ""
    host       = "somewhere.org"
    path       = "/somedir/git.invalid.infradead.org.foo.mtd-utils.git"
    parameters = {"protocol": "http", "branch": "master_backup"}

Then the BitBake will make the actual replacements with a re.sub(pattern.part, replacement.part, upstream.part, 1) command for the all parts except the parameters. This means that if you want to replace a part complitely, then pattern should be a ".*" instead of a "". For the parameters the new keys will be added to the list and the old values will be replaced. As the result we will get as follows:

    scheme     = "git"
    user       = ""
    password   = ""
    host       = "somewhere.org"
    path       = "/somedir/git.invalid.infradead.org.foo.mtd-utils.git"
    parameters = {"branch": "master_backup", "protocol": "http", "tag": "1234567890123456789012345678901234567890"}

On the last step the BitBake will check that the result.path finished with a basename of the upstream.path. Since the result.scheme and the upstream.scheme are same, the basename will be defined as an "mtd-utils.git" (if a scheme will be different, then basename whould be defined to a "git2_git.invalid.infradead.org.foo.mtd-utils.git.tar.gz").

When the last check passes, the BitBake will combine a result to a new mirror URI that will be used as an alternative source for the files: git://somewhere.org/somedir/git.invalid.infradead.org.foo.mtd-utils.git;branch=master_backup;protocol=http;tag=1234567890123456789012345678901234567890.

You can find additional replacements examples in the BitBake unit tests code[3].

Used Documentation

[1]: https://www.yoctoproject.org/docs/current/ref-manual/ref-manual.html#var-PREMIRRORS ‘Simple example’

[2]: https://www.yoctoproject.org/docs/current/mega-manual/mega-manual.html#var-SSTATE_MIRRORS ‘extended example’

[3]: https://github.com/openembedded/bitbake/blob/master/lib/bb/tests/fetch.py#L374 ‘Additional replacement examples’

[4]: https://github.com/openembedded/bitbake/blob/master/lib/bb/fetch2/__init__.py#L904 ‘Bitbake mirror mechanism implementation’

© 2015-2020 — Nikolai Merinov