Gentoo Archives: gentoo-pms

From: Ulrich Mueller <ulm@g.o>
To: gentoo-pms <gentoo-pms@l.g.o>
Cc: gentoo-dev <gentoo-dev@l.g.o>, Gentoo Prefix <prefix@g.o>, James Le Cuirot <chewi@g.o>
Subject: [gentoo-pms] [PMS] [PATCH v2] Correct the definition of ESYSROOT as EPREFIX isn't always applicable
Date: Sun, 21 Jun 2020 13:02:02
In Reply to: [gentoo-pms] [PMS] [PATCH] Correct the definition of ESYSROOT as EPREFIX isn't always applicable by James Le Cuirot
1 Coming back to this old thread (from July 2019). After some discussion
2 in #gentoo-pms today, here is an updated version of the patch.
4 Cross-posting to gentoo-dev and CCing prefix again, to make sure that
5 everyone is on the same page.
7 Ulrich
10 From 712772b7ef5543693147d8f96c03189e810a6ee8 Mon Sep 17 00:00:00 2001
11 From: James Le Cuirot <chewi@g.o>
12 Date: Sun, 28 Jul 2019 22:37:35 +0100
13 Subject: [PATCH] Correct the definition of ESYSROOT as EPREFIX isn't always
14 applicable
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19 It was originally envisaged (but not stated in PMS) that SYSROOT would
20 only ever need to equal / or ROOT as a distinct SYSROOT would have no
21 benefit. A check was added to Portage to ensure this held. Myself, the
22 ChromiumOS team, and others have since been caught out by this check
23 when trying to bootstrap brand new systems from scratch. You cannot
24 bootstrap with no headers at all! The check will therefore be adjusted
25 to merely ensure that SYSROOT is / when ROOT is /.
27 There were differing assumptions about how prefixes applied to the
28 above. EPREFIX is traditionally something the user sets so some
29 thought that it would be applied to SYSROOT, regardless of the
30 latter's value. In order to honor the rule about there being no
31 distinct SYSROOT, this would mean that if SYSROOT is / then EPREFIX
32 would have to match BROOT. Despite that limitation, ESYSROOT was
33 written into PMS with a fixed value of ${SYSROOT}${EPREFIX}. Being
34 somewhat unfamiliar with prefix at the time, I didn't realise that
35 this view didn't align with what I'd had in mind and it was only when
36 I came to need a distinct SYSROOT that I realised there was a problem.
38 crossdev toolchains are installed to ${EPREFIX}/usr/${CHOST} but have
39 no further prefix appended and packages subsequently installed with
40 cross-emerge are placed in this location by setting ROOT. Bug #642604
41 recently revealed that the build system's prefix was being erroneously
42 duplicated on the end but I have now fixed this.
44 What if we want to bootstrap a brand new prefixed system using the
45 crossdev system as SYSROOT? This is the distinct SYSROOT case. The
46 problem is that there is no distinct variable for SYSROOT's prefix
47 and, as already stated, ESYSROOT is always ${SYSROOT}${EPREFIX}. We
48 therefore cannot do it! If the crossdev prefix is blank then ROOT's
49 must be blank too.
51 I also never intended to have the aforementioned limitation where
52 EPREFIX must match BROOT when SYSROOT is /. These are both entirely
53 artificial restrictions.
55 So how should it work instead? We originally intended for SYSROOT to
56 equal either / or ROOT so I imagined the prefix would automatically be
57 adjusted to match the prefix applicable at the matching location,
58 namely BROOT or EPREFIX. This is obviously more flexible than forcing
59 it to match EPREFIX.
61 What about the distinct SYSROOT case? With no distinct variable, we
62 have no way to explicitly set a prefix but this is likely only needed
63 when bootstrapping against crossdev systems, which are unprefixed by
64 nature. We therefore simply assume that the prefix is blank in this
65 case.
67 What about the cross-prefix case? Here, SYSROOT matches both / and
68 ROOT so which prefix do we choose? The script sets
69 flags to build against the target prefix so EPREFIX is used in this
70 case. This happens to fit the current definition of ESYSROOT anyway.
72 Legitimate concerns have been raised about building for a system with
73 a different prefix to the one you're building against. The only
74 binaries that leak from SYSROOT to ROOT are static libraries. Headers
75 from SYSROOT will obviously also influence how ROOT's binaries are
76 built. It is entirely possible that SYSROOT's prefix may leak through
77 a header but grepping /usr/include on my own main system reveals only
78 a few paths from a small handful of packages. pkg-config files
79 invariably include paths but these are almost always used at build
80 time, not runtime. A differing prefix would likely only occur in cases
81 involving core packages like the libc and kernel headers anyway. Also
82 consider that we have never prevented this from happening in the
83 past. It has always been possible to do "EPREFIX=/foo emerge bar" from
84 some system with a different prefix or no prefix at all. All we're
85 doing here is including the prefix (if any) in the ESYSROOT variable.
87 Should this warrant a new EAPI? I don't think so. All existing usage
88 of ESYSROOT that I have seen still fits with this new definition and
89 most of that usage has come from me. We're not even changing what the
90 variable is used for, just loosening the constraints around what it
91 can be set to.
93 If you have doubts about whether this makes sense or actually works in
94 practise, I have experimented with a prefixed system using all the
95 different combinations I could think of, including cross-compiling,
96 and it all worked as expected. Keep in mind that ESYSROOT is not magic
97 and currently isn't used very much. As such, neither the toolchain nor
98 pkg-config will use this sysroot if you don't explicitly tell them
99 to. For the former, I find CC="${CHOST}-gcc --sysroot=${ESYSROOT}"
100 works well. For the latter, crossdev installs a cross-pkg-config
101 wrapper but it is completely lacking prefix support at the moment. I
102 have fixes waiting on this change.
104 Signed-off-by: James Le Cuirot <chewi@g.o>
105 Signed-off-by: Ulrich Müller <ulm@g.o>
106 ---
107 dependencies.tex | 16 +++++++++++++++-
108 ebuild-env-vars.tex | 6 +++---
109 2 files changed, 18 insertions(+), 4 deletions(-)
111 diff --git a/dependencies.tex b/dependencies.tex
112 index 3305b86..8bfe6f3 100644
113 --- a/dependencies.tex
114 +++ b/dependencies.tex
115 @@ -36,13 +36,27 @@
116 \midrule
117 Binary compatible with & \t{CBUILD} & \t{CHOST} & \t{CHOST} \\
118 Base unprefixed path & \t{/} & \t{\$\{SYSROOT\}} & \t{\$\{ROOT\}} \\
119 - Relevant offset-prefix & \t{\$\{BROOT\}} & \t{\$\{EPREFIX\}} & \t{\$\{EPREFIX\}} \\
120 + Relevant offset-prefix & \t{\$\{BROOT\}} & See table~\ref{tab:depend-prefix}
121 + & \t{\$\{EPREFIX\}} \\
122 Path combined with prefix & \t{\$\{BROOT\}} & \t{\$\{ESYSROOT\}} & \t{\$\{EROOT\}} \\
123 PM query command option & \t{-b} & \t{-d} & \t{-r} \\
124 \bottomrule
125 \end{tabular}
126 \end{centertable}
128 +\begin{centertable}{Prefix values for \t{DEPEND}}
129 + \label{tab:depend-prefix}
130 + \begin{tabular}{llll}
131 + \toprule
132 + If \t{SYSROOT} is: & \t{\$\{ROOT\}} & Empty but not equal to \t{\$\{ROOT\}}
133 + & Other \\
134 + \midrule
135 + Then offset-prefix is: & \t{\$\{EPREFIX\}} & \t{\$\{BROOT\}} & Blank \\
136 + And \t{ESYSROOT} is: & \t{\$\{EROOT\}} & \t{\$\{BROOT\}} & \t{\$\{SYSROOT\}} \\
137 + \bottomrule
138 + \end{tabular}
139 +\end{centertable}
140 +
141 There are three classes of dependencies supported by ebuilds:
143 \begin{compactitem}
144 diff --git a/ebuild-env-vars.tex b/ebuild-env-vars.tex
145 index ae8bd7d..117a19d 100644
146 --- a/ebuild-env-vars.tex
147 +++ b/ebuild-env-vars.tex
148 @@ -141,9 +141,9 @@ variable.
149 \t{ESYSROOT} &
150 Ditto &
151 No &
152 - Contains the concatenation of the paths in the \t{SYSROOT} and \t{EPREFIX} variables,
153 - for convenience. See also the \t{EPREFIX} variable. Only for EAPIs listed
154 - in table~\ref{tab:offset-env-vars-table} as supporting \t{ESYSROOT}. \\
155 + Contains the concatenation of the \t{SYSROOT} path and applicable prefix value, as determined
156 + by table~\ref{tab:depend-prefix}. Only for EAPIs listed in table~\ref{tab:offset-env-vars-table}
157 + as supporting \t{ESYSROOT}. \\
158 \t{BROOT} &
159 Ditto &
160 No &
161 --
162 2.27.0


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