[bug#29271] [PATCH 1/1] doc: Split the "Substitutes" section into subsections.

[Chris Marusich]

* doc/guix.texi (Substitutes): Move information into sections, and create one
  new subsection regarding substitution failure.
  (Top): In the detailed listing, add entries for the new subsections.
  (Hydra): Move info about Hydra here.  Add information that explains when
  substitutes are enabled by default.
  (Substitute Server Authorization): Move information about how to enable or
  disable substitutes here.
  (Substitute Authentication): Move information about how Guix authenticates
  substitutes here.
  (Proxy Settings): Move information about how to download substitutes via
  proxy here.
  (Substitution Failure): New section.  Add information about how Guix handles
  substitution failure.  Be extra clear about behavior concerning --fallback.
  (On Trusting Binaries): Move information about trusting binaries here.
  (Invoking guix-daemon): Remove an incorrect statement about when substitutes
  are enabled by default.
  (Common Build Options): Clarify the behavior of --fallback.

Co-authored-by: myglc2 <address@hidden>
---
 doc/guix.texi | 126 +++++++++++++++++++++++++++++++++++++++++++---------------
 1 file changed, 94 insertions(+), 32 deletions(-)

diff --git a/doc/guix.texi b/doc/guix.texi
index 11a9de689..404ea2406 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -130,6 +130,15 @@ Package Management
 * Invoking guix pack::          Creating software bundles.
 * Invoking guix archive::       Exporting and importing store files.
 
+Substitutes
+
+* Hydra::                           One particular source of substitutes.
+* Substitute Server Authorization:: How to enable or disable substitutes.
+* Substitute Authentication::       How Guix verifies substitutes.
+* Proxy Settings::                  How to get substitutes via proxy.
+* Substitution Failure::            What happens when substitution fails.
+* On Trusting Binaries::            How can you trust that binary blob?
+
 Programming Interface
 
 * Defining Packages::           Defining new packages.
@@ -1108,10 +1117,6 @@ Do not use substitutes for build products.  That is, 
always build things
 locally instead of allowing downloads of pre-built binaries
 (@pxref{Substitutes}).
 
-By default substitutes are used, unless the client---such as the
address@hidden package} command---is explicitly invoked with
address@hidden
-
 When the daemon runs with @code{--no-substitutes}, clients can still
 explicitly enable substitution @i{via} the @code{set-build-options}
 remote procedure call (@pxref{The Store}).
@@ -2108,7 +2113,6 @@ preserve transformations across upgrades, you should 
define your own
 package variant in a Guile module and add it to @code{GUIX_PACKAGE_PATH}
 (@pxref{Defining Packages}).
 
-
 @node Substitutes
 @section Substitutes
 
@@ -2116,8 +2120,8 @@ package variant in a Guile module and add it to 
@code{GUIX_PACKAGE_PATH}
 @cindex pre-built binaries
 Guix supports transparent source/binary deployment, which means that it
 can either build things locally, or download pre-built items from a
-server.  We call these pre-built items @dfn{substitutes}---they are
-substitutes for local build results.  In many cases, downloading a
+server, or both.  We call these pre-built items @dfn{substitutes}---they
+are substitutes for local build results.  In many cases, downloading a
 substitute is much faster than building things locally.
 
 Substitutes can be anything resulting from a derivation build
@@ -2125,6 +2129,20 @@ Substitutes can be anything resulting from a derivation 
build
 pre-built package binaries, but source tarballs, for instance, which
 also result from derivation builds, can be available as substitutes.
 
address@hidden
+* Hydra::                           One particular source of substitutes.
+* Substitute Server Authorization:: How to enable or disable substitutes.
+* Substitute Authentication::       How Guix verifies substitutes.
+* Proxy Settings::                  How to get substitutes via proxy.
+* Substitution Failure::            What happens when substitution fails.
+* On Trusting Binaries::            How can you trust that binary blob?
address@hidden menu
+
address@hidden Hydra
address@hidden Hydra
+
address@hidden hydra
address@hidden build farm
 The @code{hydra.gnu.org} server is a front-end to a build farm that
 builds packages from the GNU distribution continuously for some
 architectures, and makes them available as substitutes.  This is the
@@ -2141,9 +2159,19 @@ using HTTP makes all communications visible to an 
eavesdropper, who
 could use the information gathered to determine, for instance, whether
 your system has unpatched security vulnerabilities.
 
+Substitutes from Hydra are enabled by default when using the Guix System
+Distribution (@pxref{GNU Distribution}).  However, substitutes are
+disabled by default when using Guix on a foreign distribution
+(@pxref{Installation}).  The following paragraphs describe how to enable
+or disable substitutes for Hydra; the same procedure can also be used to
+enable substitutes for any other substitute server.
+
+
address@hidden Substitute Server Authorization
address@hidden Substitute Server Authorization
+
 @cindex security
address@hidden digital signatures
address@hidden substitutes, authorization thereof
address@hidden substitutes, how to enable
 @cindex access control list (ACL), for substitutes
 @cindex ACL (access control list), for substitutes
 To allow Guix to download substitutes from @code{hydra.gnu.org} or a
@@ -2153,7 +2181,7 @@ imports, using the @command{guix archive} command 
(@pxref{Invoking guix
 archive}).  Doing so implies that you trust @code{hydra.gnu.org} to not
 be compromised and to serve genuine substitutes.
 
-This public key is installed along with Guix, in
+Hydra's public key is installed along with Guix, in
 @address@hidden/share/guix/hydra.gnu.org.pub}, where @var{prefix} is
 the installation prefix of Guix.  If you installed Guix from source,
 make sure you checked the GPG signature of
@@ -2194,6 +2222,17 @@ The following files would be downloaded:
 This indicates that substitutes from @code{hydra.gnu.org} are usable and
 will be downloaded, when possible, for future builds.
 
address@hidden substitutes, how to disable
+The substitute mechanism can be disabled globally by running
address@hidden with @code{--no-substitutes} (@pxref{Invoking
+guix-daemon}).  It can also be disabled temporarily by passing the
address@hidden option to @command{guix package}, @command{guix
+build}, and other command-line tools.
+
address@hidden Substitute Authentication
address@hidden Substitute Authentication
+
address@hidden digital signatures
 Guix detects and raises an error when attempting to use a substitute
 that has been tampered with.  Likewise, it ignores substitutes that are
 not signed, or that are not signed by one of the keys listed in the ACL.
@@ -2218,15 +2257,6 @@ comes first in the list and can be considered a mirror of
 produce the same binaries, thanks to bit-reproducible builds (see
 below).
 
address@hidden http_proxy
-Substitutes are downloaded over HTTP or HTTPS.
-The @code{http_proxy} environment
-variable can be set in the environment of @command{guix-daemon} and is
-honored for downloads of substitutes.  Note that the value of
address@hidden in the environment where @command{guix build},
address@hidden package}, and other client commands are run has
address@hidden no effect}.
-
 When using HTTPS, the server's X.509 certificate is @emph{not} validated
 (in other words, the server is not authenticated), contrary to what
 HTTPS clients such as Web browsers usually do.  This is because Guix
@@ -2234,18 +2264,49 @@ authenticates substitute information itself, as 
explained above, which
 is what we care about (whereas X.509 certificates are about
 authenticating bindings between domain names and public keys.)
 
-You can get statistics on the substitutes provided by a server using the
address@hidden weather} command (@pxref{Invoking guix weather}).
-
-The substitute mechanism can be disabled globally by running
address@hidden with @code{--no-substitutes} (@pxref{Invoking
-guix-daemon}).  It can also be disabled temporarily by passing the
address@hidden option to @command{guix package}, @command{guix
-build}, and other command-line tools.
-
address@hidden Proxy Settings
address@hidden Proxy Settings
 
address@hidden On Trusting Binaries
address@hidden http_proxy
+Substitutes are downloaded over HTTP or HTTPS.
+The @code{http_proxy} environment
+variable can be set in the environment of @command{guix-daemon} and is
+honored for downloads of substitutes.  Note that the value of
address@hidden in the environment where @command{guix build},
address@hidden package}, and other client commands are run has
address@hidden no effect}.
 
address@hidden Substitution Failure
address@hidden Substitution Failure
+
+Even when a substitute for a derivation is available, sometimes the
+substitution attempt will fail.  This can happen for a variety of
+reasons: the substitute server might be offline, the substitute may
+recently have been deleted, the connection might have been interrupted,
+etc.
+
+When substitutes are enabled and a substitute for a derivation is
+available, but the substitution attempt fails, Guix will attempt to
+build the derivation locally depending on whether or not
address@hidden was given (@pxref{fallback-option}).  Specifically, if
address@hidden was omitted, then no local build will be performed,
+and the derivation is considered to have failed.  However, if
address@hidden was given, then Guix will attempt to build the
+derivation locally, and the success or failure of the derivation depends
+on the success or failure of the local build.  Note that when
+substitutes are disabled or no substitute is available for the
+derivation in question, a local build will @emph{always} be performed,
+regardless of whether or not @code{--fallback} was given.
+
+To get an idea of how many substitutes are available right now, you can
+try running the @command{guix weather} command (@pxref{Invoking guix
+weather}).  This command provides statistics on the substitutes provided
+by a server.
+
address@hidden On Trusting Binaries
address@hidden On Trusting Binaries
+
address@hidden trust, of pre-built binaries
 Today, each individual's control over their own computing is at the
 mercy of institutions, corporations, and groups with enough power and
 determination to subvert the computing infrastructure and exploit its
@@ -2272,7 +2333,6 @@ In the future, we want Guix to have support to publish 
and retrieve
 binaries to/from other users, in a peer-to-peer fashion.  If you would
 like to discuss this project, join us on @email{guix-devel@@gnu.org}.
 
-
 @node Packages with Multiple Outputs
 @section Packages with Multiple Outputs
 
@@ -5191,9 +5251,11 @@ derivations has failed.
 @itemx -n
 Do not build the derivations.
 
address@hidden
 @item --fallback
-When substituting a pre-built binary fails, fall back to building
-packages locally.
+Attempt to build locally instead of issuing an error when substitutes
+are enabled and a substitute is available, but the substitution attempt
+fails (@pxref{Substitution Failure}).
 
 @item address@hidden
 @anchor{client-substitute-urls}
-- 
2.14.2