vrt: Turn hdr_t into a structured type

[dridi]

Without changing the memory layout of our pseudo-pascal-string for header names, this structured alternative clarifies the semantics and removes error-prone pointer arithmetics spread out everywhere.

The new structure adds type safety to this specific use case.

It comes with a new static analyzer with the HDR() macro, and a CAST_HDR() macro that includes a check with the CHECK_HDR() macro also introduced by this patch series.

Solves #4304
Closes #3215

[nigoroll]

👍
all of bugwash approved

[bsdphk]

looks good to me

[dridi]

It comes with a new static analyzer with the HDR() macro,

I meant of course static initializer... I will fix that before merging.

[dridi]

Maybe before I get around to fixing the commit message someone™ could run flexelint and see how it fares?

[nigoroll]

run flexelint

Count   Type    Number  Text

4       Info    740     Unusual pointer cast (incompatible indirect types)
8       Info    754     local ___ member '___' (___) not referenced
1       Info    843     Variable '___' (___) could be declared as const
--- _.fl.old	2025-04-09 17:35:06.597578158 +0200
+++ _.fl	2025-04-09 17:36:50.401846332 +0200
@@ -41,6 +41,25 @@
 --- Module:   cache/cache_expire.c (C)
 
 --- Module:   cache/cache_fetch.c (C)
+File cache/cache_fetch.c, Line 67
+                                          _
+static hdr_t H_X_Varnish = HDR("X-Varnish");
+    Info 740: Unusual pointer cast (incompatible indirect types)
+
+    --- Wrap-up for Module: cache/cache_fetch.c
+
+    Info 843: Variable 'H_X_Varnish' (line 67, file cache/cache_fetch.c) could
+    be declared as const
+File cache/cache_fetch.c, Line 67
+    Info 830: Location cited in prior message
+    Info 754: local struct member '_l' (line 67, file cache/cache_fetch.c) not
+    referenced
+File cache/cache_fetch.c, Line 67
+    Info 830: Location cited in prior message
+    Info 754: local struct member '_s' (line 67, file cache/cache_fetch.c) not
+    referenced
+File cache/cache_fetch.c, Line 67
+    Info 830: Location cited in prior message
 
 --- Module:   cache/cache_fetch_proc.c (C)
 
@@ -49,6 +68,45 @@
 --- Module:   cache/cache_hash.c (C)
 
 --- Module:   cache/cache_http.c (C)
+File cache/cache_http.c, Line 60
+                               _
+hdr_t H__Status	= HDR(":status");
+    Info 740: Unusual pointer cast (incompatible indirect types)
+File cache/cache_http.c, Line 61
+                              _
+hdr_t H__Proto	= HDR(":proto");
+    Info 740: Unusual pointer cast (incompatible indirect types)
+File cache/cache_http.c, Line 62
+                               _
+hdr_t H__Reason	= HDR(":reason");
+    Info 740: Unusual pointer cast (incompatible indirect types)
+
+    --- Wrap-up for Module: cache/cache_http.c
+
+    Info 754: local struct member '_l' (line 60, file cache/cache_http.c) not
+    referenced
+File cache/cache_http.c, Line 60
+    Info 830: Location cited in prior message
+    Info 754: local struct member '_s' (line 60, file cache/cache_http.c) not
+    referenced
+File cache/cache_http.c, Line 60
+    Info 830: Location cited in prior message
+    Info 754: local struct member '_l' (line 61, file cache/cache_http.c) not
+    referenced
+File cache/cache_http.c, Line 61
+    Info 830: Location cited in prior message
+    Info 754: local struct member '_s' (line 61, file cache/cache_http.c) not
+    referenced
+File cache/cache_http.c, Line 61
+    Info 830: Location cited in prior message
+    Info 754: local struct member '_l' (line 62, file cache/cache_http.c) not
+    referenced
+File cache/cache_http.c, Line 62
+    Info 830: Location cited in prior message
+    Info 754: local struct member '_s' (line 62, file cache/cache_http.c) not
+    referenced
+File cache/cache_http.c, Line 62
+    Info 830: Location cited in prior message
 
 --- Module:   cache/cache_lck.c (C)
 
@@ -990,3 +1048,6 @@
 
 Count   Type    Number  Text
 
+4       Info    740     Unusual pointer cast (incompatible indirect types)
+8       Info    754     local ___ member '___' (___) not referenced
+1       Info    843     Variable '___' (___) could be declared as const

[nigoroll]

I am working on a suggestion to make Flexelint happy(er).
edit: For now, I have not found a nice way for the "local member not referenced" warning (not just silencing it)