[BUG] Using --input-spec-root-directory fails

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?

Tested with https://oss.sonatype.org/content/repositories/snapshots/org/openapitools/openapi-generator-cli/7.11.0-SNAPSHOT/openapi-generator-cli-7.11.0-20241218.082146-58.jar

• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

Using --input-spec-root-directory fails with exception.

PS F:\src\app> java -jar .\openapi-generator-cli.jar generate --input-spec-root-directory .\app\specs -g csharp -o generated               
[main] INFO  o.o.codegen.config.MergedSpecBuilder - In spec root directory .\app\specs found specs [swagger.yaml]
[main] INFO  o.o.codegen.config.MergedSpecBuilder - Reading spec: .\app\specs\swagger.yaml
Merge input spec would be used - .\app\specs\_merged_spec.yaml
[main] WARN  io.swagger.v3.parser.OpenAPIV3Parser - Exception while resolving:
java.lang.NullPointerException: Cannot invoke "io.swagger.v3.oas.models.parameters.Parameter.getSchema()" because "parameter" is null
        at io.swagger.v3.parser.processors.ExternalRefProcessor.lambda$processRefToExternalPathItem$0(ExternalRefProcessor.java:358)
        at java.base/java.util.stream.ReferencePipeline$2$1.accept(ReferencePipeline.java:196)
        at java.base/java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1709)
        at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:570)
        at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:560)
        at java.base/java.util.stream.ForEachOps$ForEachOp.evaluateSequential(ForEachOps.java:151)
        at java.base/java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(ForEachOps.java:174)
        at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:265)
        at java.base/java.util.stream.ReferencePipeline.forEach(ReferencePipeline.java:636)
        at io.swagger.v3.parser.processors.ExternalRefProcessor.processRefToExternalPathItem(ExternalRefProcessor.java:359)
        at io.swagger.v3.parser.processors.PathsProcessor.processReferencePath(PathsProcessor.java:325)
        at io.swagger.v3.parser.processors.PathsProcessor.processPaths(PathsProcessor.java:65)
        at io.swagger.v3.parser.OpenAPIResolver.resolve(OpenAPIResolver.java:72)
        at io.swagger.v3.parser.OpenAPIResolver.resolve(OpenAPIResolver.java:59)
        at io.swagger.v3.parser.OpenAPIV3Parser.resolve(OpenAPIV3Parser.java:240)
        at io.swagger.v3.parser.OpenAPIV3Parser.readContents(OpenAPIV3Parser.java:183)
        at io.swagger.v3.parser.OpenAPIV3Parser.readLocation(OpenAPIV3Parser.java:97)
        at io.swagger.parser.OpenAPIParser.readLocation(OpenAPIParser.java:16)
        at org.openapitools.codegen.config.CodegenConfigurator.toContext(CodegenConfigurator.java:686)
        at org.openapitools.codegen.config.CodegenConfigurator.toClientOptInput(CodegenConfigurator.java:744)
        at org.openapitools.codegen.cmd.Generate.execute(Generate.java:527)
        at org.openapitools.codegen.cmd.OpenApiGeneratorCommand.run(OpenApiGeneratorCommand.java:32)
        at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:66)
Exception in thread "main" org.openapitools.codegen.SpecValidationException: There were issues with the specification. The option can be disabled via validateSpec (Maven/Gradle) or --skip-validate-spec (CLI).
 | Error count: 1, Warning count: 0
Errors:
        -Cannot invoke "io.swagger.v3.oas.models.parameters.Parameter.getSchema()" because "parameter" is null

        at org.openapitools.codegen.config.CodegenConfigurator.toContext(CodegenConfigurator.java:717)
        at org.openapitools.codegen.config.CodegenConfigurator.toClientOptInput(CodegenConfigurator.java:744)
        at org.openapitools.codegen.cmd.Generate.execute(Generate.java:527)
        at org.openapitools.codegen.cmd.OpenApiGeneratorCommand.run(OpenApiGeneratorCommand.java:32)
        at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:66)

openapi-generator version

Tested with https://oss.sonatype.org/content/repositories/snapshots/org/openapitools/openapi-generator-cli/7.11.0-SNAPSHOT/openapi-generator-cli-7.11.0-20241218.082146-58.jar

and 7.10.0

OpenAPI declaration file content or url

https://github.com/networknt/model-config/blob/master/rest/swagger/petstore/2.0.0/swagger.yaml

Generation Details

Windows 11

PS F:\src\> java --version
openjdk 23.0.1 2024-10-15
OpenJDK Runtime Environment Temurin-23.0.1+11 (build 23.0.1+11)
OpenJDK 64-Bit Server VM Temurin-23.0.1+11 (build 23.0.1+11, mixed mode, sharing)

PS F:\src\app> java -jar .\openapi-generator-cli.jar generate --input-spec-root-directory .\app\specs -g csharp -o generated               

Steps to reproduce

1. Download linked yaml
2. Place in folder
3. Use --input-spec-root-directory with any generator
4. Get exception

Same issue with json spec.

Related issues/PRs

Suggest a fix

Start with MergedSpecBuilder and the generation path through CodegenConfigurator.toContext, then inspect the ExternalRefProcessor stack trace while reproducing with the linked swagger.yaml and --input-spec-root-directory. Done means generation no longer fails validation with the reported null-parameter exception for the supplied YAML and JSON cases; the payload does not name a test file.