This simplifies the code since one no longer needs to think about
whether '.guix-channel' was present.
* guix/channels.scm (read-channel-metadata): Always pass a string as the
first argument to 'channel-metadata'.
(read-channel-metadata-from-source): Always return a <channel-metadata>
record.
(channel-instance-dependencies): Remove now unneeded 'match'.
(standard-module-derivation): Assume DIRECTORY is never #f and contains
a leading slash.
* tests/channels.scm (channel-metadata-directory)
(channel-metadata-dependencies): New procedures.
("channel-instance-metadata returns #f if .guix-channel does not
exist"): Remove.
("channel-instance-metadata returns default if .guix-channel does not
exist"): New test.
(make-instance): Use 'write' instead of 'display' when creating
'.guix-channel'.
(instance--no-deps): Remove dependencies.
(instance--sub-directory): New variable.
("channel-instance-metadata and default dependencies")
("channel-instance-metadata and directory"): New tests.
("latest-channel-instances excludes duplicate channel dependencies"):
Expect 'channel-commit' to return a string and adjust accordingly.
Until now the 'version' field in '.guix-channel' could be omitted, or it
could be any value.
* guix/channels.scm (read-channel-metadata): Rename to...
(channel-instance-metadata): ... this.
(channel-instance-dependencies): Adjust accordingly.
(read-channel-metadata): New procedure. Use 'match'
to require a 'version' field. Provide proper error handling when the
channel sexp is malformed or when given an unsupported version number.
(read-channel-metadata-from-source): Use 'catch' and
'system-error-errno' instead of 'file-exists?'.
* tests/channels.scm (instance--unsupported-version): New variable.
(read-channel-metadata): Rename to...
(channel-instance-metadata): ... this. Rename tests accordingly.
("channel-instance-metadata rejects unsupported version"): New test.
Previously the imported modules and extensions of EXP would be missing
from the load path of 'guix repl'.
* guix/inferior.scm (gexp->derivation-in-inferior)[script]: New
variable.
[trampoline]: Write (primitive-load #$script) to PIPE. Add #$output.
* tests/channels.scm ("channel-instances->manifest")[depends?]: Check
for requisites rather than direct references.
Adjust callers accordingly.
Previous 'channel-instance->manifest' would call
'latest-channel-derivation', which could trigger another round of
'latest-repository-commit' for no good reason.
* guix/channels.scm (resolve-dependencies): New procedure.
(channel-instance-derivations)[edges]: New variable.
[instance->derivation]: New procedure.
* tests/channels.scm (make-instance): Use 'checkout->channel-instance'
instead of 'channel-instance'.
("channel-instances->manifest"): New test.
* guix/channels.scm (<channel-metadata>): New record.
(read-channel-metadata, channel-instance-dependencies): New procedures.
(latest-channel-instances): Include channel dependencies; add optional
argument PREVIOUS-CHANNELS.
(channel-instance-derivations): Build derivation for additional channels and
add it as dependency to the channel instance derivation.
* doc/guix.texi (Channels): Add subsection "Declaring Channel Dependencies".
* tests/channels.scm: New file.
* Makefile.am (SCM_TESTS): Add it.
