chore: upgrade grpc to v1.26.0

[scotthart]

This change is 

[codecov]

Codecov Report

Merging #1167 into master will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1167      +/-   ##
==========================================
+ Coverage    95.7%   95.74%   +0.03%     
==========================================
  Files         174      174              
  Lines       13601    13602       +1     
==========================================
+ Hits        13017    13023       +6     
+ Misses        584      579       -5

Impacted Files	Coverage Δ
...anner/integration_tests/client_integration_test.cc	98.47% <0%> (-0.01%)	⬇️
google/cloud/spanner/keys.h	100% <0%> (ø)	⬆️
google/cloud/spanner/samples/samples.cc	86.44% <0%> (+0.28%)	⬆️
google/cloud/spanner/client.cc	97.41% <0%> (+0.86%)	⬆️
...loud/spanner/internal/partial_result_set_resume.cc	95.45% <0%> (+4.54%)	⬆️

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[coryan]

Do we understand why the vptr change is not needed here? Maybe because we do not define any gRPC servers?

Reviewed 2 of 2 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved

[scotthart]

Do we understand why the vptr change is not needed here? Maybe because we do not define any gRPC servers?

In the bigtable library we use/pass arguments of type grpc::ServerContext* in multiple places. This class directly inherits from grpc_impl::ServerContextBase, which had one method become virtual in the new version, v1.26.0. The ubsan errors observed from the bigtable repo were:

undefined reference to 'typeinfo for grpc_impl::ServerContextBase'

In spanner, we only use grpc::GenericServerContext, once, as part of our testing. grpc::GenericServerContext inherits from grpc::ServerContext

[coryan]

Reviewable status: complete! all files reviewed, all discussions resolved

[coryan]

FWIW, the 1.26.x branch allegedly has fixes for the problems you are seeing.

[devbww]

This looks like it is probably obsolete after the recent merge of #1232.