fix(openssl/windows): report through the log, not through the exit code

Second windows run, with the instrumentation from the previous commit:

  [mcpp] call failed: path.absolute(srcroot) -> attempt to call a nil value
  [mcpp] wrote ...\mcpp_openssl_build.bat; running it
  [mcpp] batch returned; ok=true err=nil
  [mcpp]    (no lib/ directory was produced)

Two findings, both acted on:

1. path.absolute() is not in the xlings sandbox either — the `safe` helper
   caught it and fell back, which is what let the run continue at all. The call
   is now gone rather than guarded: pkginfo.install_file() already returns an
   absolute path, so srcroot derived from it is absolute too. What the path DOES
   need is separator normalisation — it arrives with '/' and '\' mixed, and cmd
   wants backslashes.

2. os.exec reported success for a batch that produced nothing and wrote nothing
   to the log. That channel cannot be trusted here, so the batch no longer
   communicates through its exit code: every step echoes into the log BEFORE
   running, and the script always exits 0 after recording RESULT=<code>, which
   Lua reads back and decides on. os.exec's return is kept but demoted to an
   advisory log line.

The batch is also written with plain \n now. io.writefile already produces CRLF
on windows, so emitting \r\n gave \r\r\n — a batch whose stray CR lands inside
`set` values and breaks parsing in exactly the way observed: no output, no
work, and a clean exit.

Author: Sunrisepeak

pkgs/c/compat.openssl.lua

@@ -447,41 +447,74 @@ local function _install_windows_impl()
         for _, f in ipairs(entries) do note("   " .. tostring(f)) end
         return false
     end
-    srcroot = safe("path.absolute(srcroot)", function() return path.absolute(srcroot) end, srcroot)
+    -- path.absolute() is NOT in the xlings sandbox (verified: "attempt to call
+    -- a nil value"), and it is not needed — pkginfo.install_file() already
+    -- returns an absolute path, so srcroot derived from it is absolute too.
+    -- cmd wants backslashes; the path arrives with both separators mixed.
+    srcroot = tostring(srcroot):gsub("/", "\\")
     note("srcroot=" .. tostring(srcroot))
 
     -- vswhere is installed with every VS 2017+ at a fixed location, and is the
     -- supported way to find the toolset; hardcoding a VS path breaks on the
     -- next release. `-products *` is required or Build Tools-only machines
     -- (which is what CI images often are) report nothing.
+    -- The batch reports through the LOG, not through its exit code: the first
+    -- attempt came back ok=true from os.exec while having produced nothing and
+    -- written nothing, so that channel cannot be trusted here. Every step
+    -- announces itself into the log BEFORE running, and the script always
+    -- exits 0 after recording RESULT=<code>, which is what Lua then reads.
+    --
+    -- Written with plain \n: io.writefile on windows already produces CRLF, and
+    -- emitting \r\n here would give \r\r\n — a batch file whose stray CR ends
+    -- up inside `set` values and breaks parsing in ways that look like nothing
+    -- happened at all.
+    local logw  = tostring(logf):gsub("/", "\\")
+    local prefw = tostring(prefix):gsub("/", "\\")
     io.writefile(bat, table.concat({
         "@echo off",
-        "setlocal",
+        'echo [bat] started >> "' .. logw .. '" 2>&1',
         'set "VSWHERE=%ProgramFiles(x86)%\\Microsoft Visual Studio\\Installer\\vswhere.exe"',
-        'if not exist "%VSWHERE%" exit /b 10',
-        'for /f "usebackq tokens=*" %%i in (`"%VSWHERE%" -latest -products * ' ..
-            '-requires Microsoft.VisualStudio.Component.VC.Tools.x86.x64 ' ..
-            '-property installationPath`) do set "VSPATH=%%i"',
-        'if not defined VSPATH exit /b 11',
+        'echo [bat] vswhere=%VSWHERE% >> "' .. logw .. '" 2>&1',
+        'if not exist "%VSWHERE%" ( echo [bat] RESULT=10 vswhere missing >> "' .. logw .. '" & exit /b 0 )',
+        'for /f "usebackq tokens=*" %%i in (`"%VSWHERE%" -latest -products * -requires Microsoft.VisualStudio.Component.VC.Tools.x86.x64 -property installationPath`) do set "VSPATH=%%i"',
+        'echo [bat] vspath=%VSPATH% >> "' .. logw .. '" 2>&1',
+        'if not defined VSPATH ( echo [bat] RESULT=11 no VC toolset >> "' .. logw .. '" & exit /b 0 )',
         'set "VCVARS=%VSPATH%\\VC\\Auxiliary\\Build\\vcvars64.bat"',
-        'if not exist "%VCVARS%" exit /b 12',
-        'call "%VCVARS%" >nul || exit /b 13',
-        'cd /d "' .. srcroot .. '" || exit /b 14',
-        '(perl --version && nmake /? ) >> "' .. logf .. '" 2>&1',
-        'perl Configure VC-WIN64A no-shared no-tests no-apps no-engine no-dso ' ..
-            '--prefix="' .. prefix .. '" --openssldir="' .. prefix .. '\\ssl" ' ..
-            '>> "' .. logf .. '" 2>&1 || exit /b 20',
-        'nmake >> "' .. logf .. '" 2>&1 || exit /b 21',
-        'nmake install_sw >> "' .. logf .. '" 2>&1 || exit /b 22',
+        'if not exist "%VCVARS%" ( echo [bat] RESULT=12 no vcvars64 >> "' .. logw .. '" & exit /b 0 )',
+        'call "%VCVARS%" >> "' .. logw .. '" 2>&1',
+        'if errorlevel 1 ( echo [bat] RESULT=13 vcvars failed >> "' .. logw .. '" & exit /b 0 )',
+        'echo [bat] toolset ready >> "' .. logw .. '" 2>&1',
+        'cd /d "' .. srcroot .. '"',
+        'if errorlevel 1 ( echo [bat] RESULT=14 cd failed >> "' .. logw .. '" & exit /b 0 )',
+        'where perl >> "' .. logw .. '" 2>&1',
+        'where nmake >> "' .. logw .. '" 2>&1',
+        'echo [bat] configuring >> "' .. logw .. '" 2>&1',
+        'perl Configure VC-WIN64A no-shared no-tests no-apps no-engine no-dso --prefix="' .. prefw .. '" --openssldir="' .. prefw .. '\\ssl" >> "' .. logw .. '" 2>&1',
+        'if errorlevel 1 ( echo [bat] RESULT=20 Configure failed >> "' .. logw .. '" & exit /b 0 )',
+        'echo [bat] building >> "' .. logw .. '" 2>&1',
+        'nmake >> "' .. logw .. '" 2>&1',
+        'if errorlevel 1 ( echo [bat] RESULT=21 nmake failed >> "' .. logw .. '" & exit /b 0 )',
+        'echo [bat] installing >> "' .. logw .. '" 2>&1',
+        'nmake install_sw >> "' .. logw .. '" 2>&1',
+        'if errorlevel 1 ( echo [bat] RESULT=22 nmake install_sw failed >> "' .. logw .. '" & exit /b 0 )',
+        'echo [bat] RESULT=0 >> "' .. logw .. '" 2>&1',
         "exit /b 0",
-    }, "\r\n") .. "\r\n")
+    }, "\n") .. "\n")
 
     note("wrote " .. bat .. "; running it")
-    local ok, err = pcall(os.exec, string.format('cmd /c "%s"', bat))
-    note("batch returned; ok=" .. tostring(ok) .. " err=" .. tostring(err))
-    if not ok then
+    local batw = tostring(bat):gsub("/", "\\")
+    local ok, err = pcall(os.exec, string.format('cmd /c "%s"', batw))
+    note("os.exec ok=" .. tostring(ok) .. " err=" .. tostring(err)
+         .. " (advisory only -- RESULT= in this log decides)")
+
+    local content = ""
+    local rok, rdata = pcall(io.readfile, logf)
+    if rok and rdata then content = tostring(rdata) end
+    local result = content:match("%[bat%] RESULT=(%d+)")
+    note("batch RESULT=" .. tostring(result))
+    if result ~= "0" then
         local tail = tail_lines(logf, 40) or "<no log; the failure was before the build started>"
-        log.error("%s", "compat.openssl: windows build failed (" .. tostring(err) ..
+        log.error("%s", "compat.openssl: windows build failed (RESULT=" .. tostring(result) ..
                   ")\nexit 10-13 = no Visual Studio C++ toolset found (vswhere/vcvars64), " ..
                   "20-22 = Configure/nmake failed.\nHOST REQUIREMENTS on windows: perl " ..
                   "(Strawberry Perl -- xim:perl has no windows build) and a Visual Studio " ..