From 9f162f1c93009f3a8139c4b3e72d6bbd0c1c4dce Mon Sep 17 00:00:00 2001
From: Boris Kolpackov <boris@codesynthesis.com>
Date: Tue, 7 Sep 2021 08:07:07 +0200
Subject: Add pkg-build --rebuild-checksum option

---
 bpkg/pkg-build.cli | 25 ++++++++++++++++++-------
 bpkg/pkg-build.cxx | 24 +++++++++++++++++++++++-
 2 files changed, 41 insertions(+), 8 deletions(-)

diff --git a/bpkg/pkg-build.cli b/bpkg/pkg-build.cli
index ba9bf47..f9cee12 100644
--- a/bpkg/pkg-build.cli
+++ b/bpkg/pkg-build.cli
@@ -360,6 +360,13 @@ namespace bpkg
        is otherwise a noop (performs no new package builds, upgrades, etc)."
     }
 
+    bool --no-move
+    {
+      "Don't move dependency packages between configurations. In this mode the
+       \cb{--config-*} options specify packages' current rather than new
+       locations."
+    }
+
     uint16_t --noop-exit
     {
       "<code>",
@@ -368,6 +375,17 @@ namespace bpkg
        (performs no new package builds, upgrades, etc)."
     }
 
+    string --rebuild-checksum
+    {
+      "<sum>",
+
+      "Hash the names and versions of all the packages that would be built.
+       If the resulting checksum matches the specified, then exit without
+       building anything (potentially with a special error code specified
+       with the \cb{--noop-exit} option). Otherwise, proceed to build as
+       normal. In both cases, print the resulting checksum to \cb{stdout}."
+    }
+
     uint16_t --no-private-config
     {
       "<code>",
@@ -389,13 +407,6 @@ namespace bpkg
        See \l{bpkg-cfg-create(1)} for details on linked configurations."
     }
 
-    bool --no-move
-    {
-      "Don't move dependency packages between configurations. In this mode the
-       \cb{--config-*} options specify packages' current rather than new
-       locations."
-    }
-
     dir_paths --directory|-d
     {
       "<dir>",
diff --git a/bpkg/pkg-build.cxx b/bpkg/pkg-build.cxx
index 76625e1..6661109 100644
--- a/bpkg/pkg-build.cxx
+++ b/bpkg/pkg-build.cxx
@@ -9,6 +9,7 @@
 #include <cstring>    // strlen()
 #include <iostream>   // cout
 
+#include <libbutl/sha256.mxx>
 #include <libbutl/standard-version.mxx>
 
 #include <bpkg/package.hxx>
@@ -6432,9 +6433,13 @@ namespace bpkg
     // --plan, then we print it as long as it is not empty.
     //
     string plan;
+    sha256 csum;
     bool need_prompt (false);
 
-    if (o.print_only () || !o.yes () || o.plan_specified ())
+    if (!o.yes ()           ||
+        o.print_only ()     ||
+        o.plan_specified () ||
+        o.rebuild_checksum_specified ())
     {
       bool first (true); // First entry in the plan.
 
@@ -6543,6 +6548,12 @@ namespace bpkg
           string rb;
           if (!p.user_selection ())
           {
+            // Note: if we are ever tempted to truncate this, watch out for
+            // the --rebuild-checksum functionality which uses this. But then
+            // it's not clear this information is actually important: can a
+            // dependent-dependency structure change without any of the
+            // package versions changing? Doesn't feel like it should.
+            //
             for (const config_package& cp: p.required_by)
               rb += (rb.empty () ? " " : ", ") + cp.string ();
 
@@ -6579,9 +6590,20 @@ namespace bpkg
           // Print indented for better visual separation.
           //
           plan += (plan.empty () ? "  " : "\n  ") + act;
+
+        if (o.rebuild_checksum_specified ())
+          csum.append (act);
       }
     }
 
+    if (o.rebuild_checksum_specified ())
+    {
+      cout << csum.string () << endl;
+
+      if (o.rebuild_checksum () == csum.string ())
+        return o.noop_exit_specified () ? o.noop_exit () : 0;
+    }
+
     if (o.print_only ())
       return 0;
 
-- 
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