Since custom handlers are not copied to the regeneration lambda, use the default handler and ignore the customer handler when building the regeneration lambda

[rsanchez]

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When you use a custom handler option (mine is handler.js, so my config is handler.handler), the custom handler file gets copied to the default lambda and the image lambda, but not the regeneration lambda. However, the regeneration lambda still attempts to load the customer handler named in the options, and since that custom handler is not copied over, the regeneration lambda fails. This is the error I see:

{
    "errorType": "Runtime.ImportModuleError",
    "errorMessage": "Error: Cannot find module 'handler'\nRequire stack:\n- /var/runtime/UserFunction.js\n- /var/runtime/Runtime.js\n- /var/runtime/index.js",
    "stack": [
        "Runtime.ImportModuleError: Error: Cannot find module 'handler'",
        "Require stack:",
        "- /var/runtime/UserFunction.js",
        "- /var/runtime/Runtime.js",
        "- /var/runtime/index.js",
        "    at _loadUserApp (/var/runtime/UserFunction.js:221:13)",
        "    at Object.module.exports.load (/var/runtime/UserFunction.js:279:17)",
        "    at Object.<anonymous> (/var/runtime/index.js:43:34)",
        "    at Module._compile (internal/modules/cjs/loader.js:1085:14)",
        "    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1114:10)",
        "    at Module.load (internal/modules/cjs/loader.js:950:32)",
        "    at Function.Module._load (internal/modules/cjs/loader.js:790:12)",
        "    at Function.executeUserEntryPoint [as runMain] (internal/modules/run_main.js:75:12)",
        "    at internal/main/run_main_module.js:17:47"
    ]
}

Here's another person experiencing the same issue: https://github.com/serverless-nextjs/serverless-next.js/issues/1098#issuecomment-873509152

This PR use the default handler name and ignores the custom handler when building the regeneration lambda.

[slsnextbot]

Handler Size Report

No changes to handler sizes.

Base Handler Sizes (kB) (commit e6367b585fb98608cd2e9327e2c8d4058ba73b00)

{
    "Lambda": {
        "Default Lambda": {
            "Standard": 1578,
            "Minified": 692
        },
        "Image Lambda": {
            "Standard": 1543,
            "Minified": 831
        }
    },
    "Lambda@Edge": {
        "Default Lambda": {
            "Standard": 1588,
            "Minified": 698
        },
        "Default Lambda V2": {
            "Standard": 1580,
            "Minified": 694
        },
        "API Lambda": {
            "Standard": 634,
            "Minified": 318
        },
        "Image Lambda": {
            "Standard": 1551,
            "Minified": 835
        },
        "Regeneration Lambda": {
            "Standard": 1233,
            "Minified": 566
        },
        "Regeneration Lambda V2": {
            "Standard": 1307,
            "Minified": 596
        }
    }
}

New Handler Sizes (kB) (commit f2940760aeaa3bd7d05d34c7f43db7771f378a21)

{
    "Lambda": {
        "Default Lambda": {
            "Standard": 1578,
            "Minified": 692
        },
        "Image Lambda": {
            "Standard": 1543,
            "Minified": 831
        }
    },
    "Lambda@Edge": {
        "Default Lambda": {
            "Standard": 1588,
            "Minified": 698
        },
        "Default Lambda V2": {
            "Standard": 1580,
            "Minified": 694
        },
        "API Lambda": {
            "Standard": 634,
            "Minified": 318
        },
        "Image Lambda": {
            "Standard": 1551,
            "Minified": 835
        },
        "Regeneration Lambda": {
            "Standard": 1233,
            "Minified": 566
        },
        "Regeneration Lambda V2": {
            "Standard": 1307,
            "Minified": 596
        }
    }
}

[codecov[bot]]

Codecov Report

Merging #2484 (f294076) into master (e6367b5) will increase coverage by 0.02%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2484      +/-   ##
==========================================
+ Coverage   83.85%   83.88%   +0.02%     
==========================================
  Files         102      102              
  Lines        3717     3716       -1     
  Branches     1191     1190       -1     
==========================================
  Hits         3117     3117              
+ Misses        588      587       -1     
  Partials       12       12              

Impacted Files	Coverage Δ
...rless-components/nextjs-component/src/component.ts	86.31% <ø> (-0.05%)	:arrow_down:
packages/libs/core/src/images/imageOptimizer.ts	84.51% <0.00%> (+0.41%)	:arrow_up:

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[rsanchez]

@telmperez does this error message relate to the issue you reported?