Provide linux-{image,headers}-generic virtual packages on most architectures

The Debian installer can choose a suitable kernel to install based on
hardware information, but image builders can't do that and they need
to know a sensible default kernel for each architecture.

I've resisted defining such a default before now, because there is no
reasonable default on some architectures.  But we can still provide a
default for those architectures where there is:

* [alpha,sparc64] default to SMP flavour; both flavours should run on
  everything but this seems more useful
* [amd64] default to amd64, which should run on everything
* [arm64] default to arm64, which should run on everything
* [armhf] default to armmp, which should run on everything; non-LPAE
  CPUs are very common
* [hppa] default to parisc, which should run on everything
* [i386] default to 686-pae, which requires PAE; non-PAE CPUs are
  quite rare
* [ia64] default to itanium, which should run on everything
* [m68k,mips64r6*,ppc64*,riscv64,s390x] default to the only flavour
* [mipsr6*] default to 32-bit flavour; 64-bit systems should use the
  64-bit architecture

The architectures with no default are:

* [armel,mips,mips64,mips64el,mipsel,sh4] flavours are platform-specific
* [powerpc] 32-bit flavour won't boot on 64-bit systems and vice versa

Closes: #960209

2 files changed, 44 insertions, 3 deletions

diff --git a/debian/bin/gencontrol.py b/debian/bin/gencontrol.py
index 9c8f7952df69..6cbb77ac8bd4 100755
--- a/debian/bin/gencontrol.py
+++ b/debian/bin/gencontrol.py
@@ -12,7 +12,7 @@ from debian_linux import config
 from debian_linux.debian import PackageDescription, PackageRelation, \
     PackageRelationEntry, PackageRelationGroup, VersionLinux
 from debian_linux.gencontrol import Gencontrol as Base, merge_packages, \
-    iter_featuresets
+    iter_featuresets, iter_flavours
 from debian_linux.utils import Templates, read_control
 
 locale.setlocale(locale.LC_CTYPE, "C.UTF-8")
@@ -334,6 +334,18 @@ class Gencontrol(Base):
         vars['localversion_headers'] = vars['localversion']
         makeflags['LOCALVERSION_HEADERS'] = vars['localversion_headers']
 
+        self.default_flavour = self.config.merge('base', arch, featureset) \
+                                          .get('default-flavour')
+        if self.default_flavour is not None:
+            if featureset != 'none':
+                raise RuntimeError("default-flavour set for %s %s,"
+                                   " but must only be set for featureset none"
+                                   % (arch, featureset))
+            if self.default_flavour \
+               not in iter_flavours(self.config, arch, featureset):
+                raise RuntimeError("default-flavour %s for %s %s does not exist"
+                                   % (self.default_flavour, arch, featureset))
+
     flavour_makeflags_base = (
         ('compiler', 'COMPILER', False),
         ('compiler-filename', 'COMPILER', True),
@@ -512,6 +524,13 @@ class Gencontrol(Base):
                 self.templates['control.headers.meta'], vars)
             assert len(packages_meta) == 2
 
+            if flavour == self.default_flavour \
+               and not self.vars['source_suffix']:
+                packages_meta[0].setdefault('Provides', PackageRelation()) \
+                                .append('linux-image-generic')
+                packages_meta[1].setdefault('Provides', PackageRelation()) \
+                                .append('linux-headers-generic')
+
             packages_own.extend(packages_meta)
 
             self.substitute_debhelper_config(
diff --git a/debian/bin/gencontrol_signed.py b/debian/bin/gencontrol_signed.py
index 3bc09c0cd6e7..75d9112cb23e 100755
--- a/debian/bin/gencontrol_signed.py
+++ b/debian/bin/gencontrol_signed.py
@@ -11,8 +11,9 @@ import subprocess
 import sys
 
 from debian_linux.config import ConfigCoreDump
-from debian_linux.debian import VersionLinux
-from debian_linux.gencontrol import Gencontrol as Base, merge_packages
+from debian_linux.debian import PackageRelation, VersionLinux
+from debian_linux.gencontrol import Gencontrol as Base, merge_packages, \
+    iter_flavours
 from debian_linux.utils import Templates, read_control
 
 
@@ -142,6 +143,19 @@ class Gencontrol(Base):
                           (arch, makeflags,
                            ' '.join(p['Package'] for p in udeb_packages))])
 
+    def do_featureset_setup(self, vars, makeflags, arch, featureset, extra):
+        self.default_flavour = self.config.merge('base', arch, featureset) \
+                                          .get('default-flavour')
+        if self.default_flavour is not None:
+            if featureset != 'none':
+                raise RuntimeError("default-flavour set for %s %s,"
+                                   " but must only be set for featureset none"
+                                   % (arch, featureset))
+            if self.default_flavour \
+               not in iter_flavours(self.config, arch, featureset):
+                raise RuntimeError("default-flavour %s for %s %s does not exist"
+                                   % (self.default_flavour, arch, featureset))
+
     def do_flavour_setup(self, vars, makeflags, arch, featureset, flavour,
                          extra):
         super(Gencontrol, self).do_flavour_setup(vars, makeflags, arch,
@@ -214,7 +228,15 @@ class Gencontrol(Base):
             for package in packages_meta:
                 del package['Build-Profiles']
 
+            if flavour == self.default_flavour \
+               and not self.vars['source_suffix']:
+                packages_meta[0].setdefault('Provides', PackageRelation()) \
+                                .append('linux-image-generic')
+                packages_meta[1].setdefault('Provides', PackageRelation()) \
+                                .append('linux-headers-generic')
+
             packages_own.extend(packages_meta)
+
             cmds_binary_arch += [
                 "$(MAKE) -f debian/rules.real install-meta "
                 "PACKAGE_NAME='%s' LINK_DOC_PACKAGE_NAME='%s' %s" %