From 798e8db974f929988200e49ada81076b406a50bd Mon Sep 17 00:00:00 2001
From: Eugene Blikh <bigbes@gmail.com>
Date: Sun, 17 May 2026 08:13:06 +0300
Subject: [PATCH] json,text: presence-tracked fields skip proto3 default
 elision
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Both codecs gated default-value elision on `f.optional or f.oneof` —
the proto3 implicit-presence shape. That elides proto2 `required`
fields set to zero and any other presence-tracked descriptor that
doesn't carry the proto3 explicit-optional flag.

Extend the bypass to `f.required` so a Cardinality{r=0} survives
the round-trip through pb.json.encode / pb.text.encode. 4 new
luatest cases pin the rule.
---
 runtime/pb/json.lua  |  8 +++++++-
 runtime/pb/text.lua  |  5 ++++-
 test/proto2_test.lua | 36 ++++++++++++++++++++++++++++++++++++
 3 files changed, 47 insertions(+), 2 deletions(-)

diff --git a/runtime/pb/json.lua b/runtime/pb/json.lua
index 28e9a6c748b2393a30434e830b763a1345ee05cb..144207847903e9214a26b3b795010c20d3c0ed83 100644
--- a/runtime/pb/json.lua
+++ b/runtime/pb/json.lua
@@ -900,7 +900,13 @@ encode_message = function(desc, t)
                 end
             else
                 local emit = true
-                if not emit_defaults and not (f.optional or f.oneof) then
+                -- Default-elision only applies to proto3 implicit-presence
+                -- shapes. Anything with explicit presence (`optional`,
+                -- oneof, proto2 `required`) emits its value verbatim — even
+                -- when it equals the type's zero/default — because the
+                -- absence of the field is observably distinct from being
+                -- set-to-default.
+                if not emit_defaults and not (f.optional or f.oneof or f.required) then
                     if f.kind == 'scalar' then
                         local pt = f.proto_type
                         if pt == 'string' or pt == 'bytes' then
diff --git a/runtime/pb/text.lua b/runtime/pb/text.lua
index dfa3d31f9fe5b52f195b53386fddf2e2a3456743..279295f05b82b0c46b1f8aefc0f604d0b9ab3fee 100644
--- a/runtime/pb/text.lua
+++ b/runtime/pb/text.lua
@@ -99,7 +99,10 @@ local function enum_token(enum_desc, v)
 end
 
 local function is_proto3_default(f, v)
-    if f.optional or f.oneof then return false end
+    -- Skip elision for any presence-tracked field — proto3 explicit
+    -- `optional`, oneof branches, and proto2 `optional`/`required` all
+    -- carry observable absence-vs-set-to-default distinctions.
+    if f.optional or f.oneof or f.required then return false end
     if f.kind == 'scalar' then
         local pt = f.proto_type
         if pt == 'string' or pt == 'bytes' then return v == '' end
diff --git a/test/proto2_test.lua b/test/proto2_test.lua
index bf01975b8c4e7f55bd6f018c3568fc45c3276dc0..f02255b365f5029e99eff5ed5df03aae1042883e 100644
--- a/test/proto2_test.lua
+++ b/test/proto2_test.lua
@@ -239,6 +239,42 @@ do
     end
 end
 
+-- JSON and text format: proto2 fields are all presence-tracked, so the
+-- elision rules built around proto3's implicit zero defaults must not
+-- fire on required-set-to-zero or optional-set-to-default.
+do
+    local g    = t.group('proto2_basic.codecs')
+    local pb   = require('pb')
+    local full = require('full.proto2_basic.proto2_basic_pb')
+
+    g.test_json_required_zero_emitted = function()
+        local s = pb.json.encode(full.Cardinality_descriptor, {r = 0})
+        local d = pb.json.decode(full.Cardinality_descriptor, s)
+        t.assert_equals(d.r, 0)
+        t.assert(s:find('"r"%s*:%s*0'), 'required int32=0 must appear in JSON: ' .. s)
+    end
+
+    g.test_json_optional_default_emitted_when_set = function()
+        local s = pb.json.encode(full.Defaults_descriptor, {i = 17, b = false})
+        t.assert(s:find('"i"'), 'optional int32 at declared default must be emitted')
+        t.assert(s:find('"b"%s*:%s*false'), 'optional bool=false must be emitted')
+    end
+
+    g.test_json_absent_optional_not_emitted = function()
+        -- Even with emit_defaults, proto2 presence-tracked fields stay
+        -- absent if the user didn't set them. We don't auto-materialize
+        -- declared defaults into JSON output.
+        local s = pb.json.encode(full.Defaults_descriptor, {}, {emit_defaults = true})
+        t.assert_not(s:find('"i"'), 'absent optional must not be emitted')
+        t.assert_not(s:find('"color"'), 'absent enum optional must not be emitted')
+    end
+
+    g.test_text_required_zero_emitted = function()
+        local s = pb.text.encode(full.Cardinality_descriptor, {r = 0})
+        t.assert(s:find('r:%s*0'), 'required int32=0 must appear in text format: ' .. s)
+    end
+end
+
 -- Parity: full and runtime modes must produce byte-identical output for
 -- the same input. Equivalent to the existing parity.full_vs_runtime group.
 do