Add support for Phased-Update-Percentage

This adds support for Phased-Update-Percentage by pinning
upgrades that are not to be installed down to 1.

The output of policy has been changed to add the level of
phasing, and documentation has been improved to document
how phased updates work.

The patch detects if it is running in a chroot, and if so, always
includes phased updates, restoring classic apt behavior to avoid
behavioral changes on buildd chroots.

Various options are added to control this all:

* APT::Get::{Always,Never}-Include-Phased-Updates and their legacy
  update-manager equivalents to always or never include phased updates
* APT::Machine-ID can be set to a UUID string to have all machines in a
  fleet phase the same
* Dir::Etc::Machine-ID is weird in that it's default is sort of like
  ../machine-id, but not really, as ../machine-id would look up
  $PWD/../machine-id and not relative to Dir::Etc; but it allows you to
  override the path to machine-id (as opposed to the value)
* Dir::Bin::ischroot is the path to the ischroot(1) binary which is used
  to detect whether we are running in a chroot.

1 files changed, 46 insertions, 2 deletions

diff --git a/apt-pkg/policy.cc b/apt-pkg/policy.cc
index 761a6ede1..bdb049ead 100644
--- a/apt-pkg/policy.cc
+++ b/apt-pkg/policy.cc
@@ -14,6 +14,7 @@
 // Include Files							/*{{{*/
 #include <config.h>
 
+#include <apt-pkg/aptconfiguration.h>
 #include <apt-pkg/cachefilter.h>
 #include <apt-pkg/configuration.h>
 #include <apt-pkg/error.h>
@@ -26,6 +27,7 @@
 #include <apt-pkg/versionmatch.h>
 
 #include <iostream>
+#include <random>
 #include <sstream>
 #include <string>
 #include <vector>
@@ -40,12 +42,17 @@ using namespace std;
 
 constexpr short NEVER_PIN = std::numeric_limits<short>::min();
 
+struct pkgPolicy::Private
+{
+   std::string machineID;
+};
+
 // Policy::Init - Startup and bind to a cache				/*{{{*/
 // ---------------------------------------------------------------------
 /* Set the defaults for operation. The default mode with no loaded policy
    file matches the V0 policy engine. */
 pkgPolicy::pkgPolicy(pkgCache *Owner) : VerPins(nullptr),
-   PFPriority(nullptr), Cache(Owner), d(NULL)
+					PFPriority(nullptr), Cache(Owner), d(new Private)
 {
    if (Owner == 0)
       return;
@@ -77,6 +84,8 @@ pkgPolicy::pkgPolicy(pkgCache *Owner) : VerPins(nullptr),
 	 CreatePin(pkgVersionMatch::Release,"",DefRel,990);
    }
    InitDefaults();
+
+   d->machineID = APT::Configuration::getMachineID();
 }
 									/*}}}*/
 // Policy::InitDefaults - Compute the default selections		/*{{{*/
@@ -274,8 +283,38 @@ void pkgPolicy::CreatePin(pkgVersionMatch::MatchType Type,string Name,
 // Policy::GetPriority - Get the priority of the package pin		/*{{{*/
 // ---------------------------------------------------------------------
 /* */
+// Returns true if this update is excluded by phasing.
+static inline bool ExcludePhased(std::string machineID, pkgCache::VerIterator const &Ver)
+{
+   // The order and fallbacks for the always/never checks come from update-manager and exist
+   // to preserve compatibility.
+   if (_config->FindB("APT::Get::Always-Include-Phased-Updates",
+		      _config->FindB("Update-Manager::Always-Include-Phased-Updates", false)))
+      return false;
+
+   if (_config->FindB("APT::Get::Never-Include-Phased-Updates",
+		      _config->FindB("Update-Manager::Never-Include-Phased-Updates", false)))
+      return true;
+
+   if (machineID.empty()			 // no machine-id
+       || getenv("SOURCE_DATE_EPOCH") != nullptr // reproducible build - always include
+       || APT::Configuration::isChroot())
+      return false;
+
+   std::string seedStr = std::string(Ver.SourcePkgName()) + "-" + Ver.VerStr() + "-" + machineID;
+   std::seed_seq seed(seedStr.begin(), seedStr.end());
+   std::minstd_rand rand(seed);
+   std::uniform_int_distribution<unsigned int> dist(0, 100);
+
+   return dist(rand) > Ver.PhasedUpdatePercentage();
+}
 APT_PURE signed short pkgPolicy::GetPriority(pkgCache::VerIterator const &Ver, bool ConsiderFiles)
 {
+   if (Ver.ParentPkg()->CurrentVer && Ver.PhasedUpdatePercentage() != 100)
+   {
+      if (ExcludePhased(d->machineID, Ver))
+	 return 1;
+   }
    if (VerPins[Ver->ID].Type != pkgVersionMatch::None)
    {
       // If all sources are never pins, the never pin wins.
@@ -454,4 +493,9 @@ bool ReadPinFile(pkgPolicy &Plcy,string File)
 }
 									/*}}}*/
 
-pkgPolicy::~pkgPolicy() {delete [] PFPriority; delete [] VerPins; }
+pkgPolicy::~pkgPolicy()
+{
+   delete[] PFPriority;
+   delete[] VerPins;
+   delete d;
+}